Download Genetics - Aurora City School District

Topic 4 and 10
Why sex????
 http://www.pbs.org/wgbh/nova/miracle/program.
html
Chromosomes
 Chromosomes are composed of DNA and protein (histone)
 Each body cell, called a somatic cell, is composed of 46
chromosomes (23 pairs).
 Each chromosome consists of two sister chromatids joined
at the centromere.
Chromosomes
• Two chromosomes composing a pair are called
homologous chromosomes (or homologues)
because they both carry genes controlling the same
inherited characteristics.
• For example, if a gene that determines whether a
person has freckles is located at a particular place, or
locus, then the other chromosome of the homologous
pair also has a gene for freckles at that locus
– However, the two homologues may have different
variations of the freckles genes (called alleles), perhaps
one that promotes freckles and one that does not.
Chromosomes
• Sex chromosome
– X and Y chromosomes
– Determine the sex of an individual, and carry genes that
perform other functions as well.
– 23rd pair of chromosomes in humans
• The two distinct chromosomes X and Y are important
exception to the general pattern of homologous
chromosomes.
• Human females have a homologous pair of X chromosomes
(XX), but males have one X and one Y chromosome (XY).
– Only small parts of the X and Y are homologous; most of the
genes carried on the X chromosome do not have counterparts
on the tiny Y, and the Y chromosome has genes lacking on the
X.
Chromosomes
 The other 22 pairs of chromosomes are autosomes, or
“body chromosomes”.
 For both autosomes and sex chromosomes, we inherit
one chromosome of each pair form our mother and
other from our father.
*a key factor in the human life cycle and in the
life cycles of all other species that reproduce
sexually.
Chromosomes
 Any cell with two homologous sets of chromosomes is
called a diploid cell, and the total number of
chromosomes is called the diploid number
(abbreviated 2n)
 For humans, the diploid number is 46; that is 2n=46
 Almost all human cells are diploid
Chromosomes
 The exception are the egg and sperm cells, collectively
known as gametes.
 Each gamete has a single set of chromosomes: 22
autosomes plus a single sex chromosome,either X or Y.
 A cell with a single chromosome set is called a haploid
cell.
 For humans, the haploid number (abbreviated n) is 23;
that is n=23
Chromosomes
 In humans, sexual intercourse allows a haploid sperm
cell from the father to reach and fuse with a haploid
egg cell of the mother in the process of fertilization.
 The resulting fertilized egg is called a zygote, which is
now diploid.
 It has two haploid sets of chromosomes: one set from the
mother and a homologous set from the father.
 The life cycle is completed as a sexually mature adult
develops from the zygote.
 Mitotic cell division ensures that all somatic cells of the
human body receive copies of all of the zygote’s 46
chromosomes.
Chromosomes
 All sexual life cycles involve an alternation of diploid
and haploid stages.
 Having haploid gametes keeps the chromosome
number from doubling in each generation.
 Gametes are made by a special sort of cell division
called meiosis, which occurs only in reproductive
organs (ovaries and testes)
 Whereas mitosis produces daughter cells with the same
numbers of chromosomes as the parent cell, meiosis
reduces the chromosome number in half.
Meiosis
 Meiosis
 Type of cell division that produces haploid gametes
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in diploid organisms.
Many of the stages of meiosis closely resemble
corresponding stages in mitosis.
Meiosis, like mitosis, is preceded by the replication
of chromosomes.
However, this single replication is followed by two
consecutive cell divisions, called Meiosis I and
Meiosis II.
These divisions result in four daughter cells, each
with a single haploid set of chromosomes.
Thus, meiosis produces daughter cells with only
half as many chromosomes as the parent cell.
Interphase
 Like mitosis, meiosis begins by interphase, during which the
chromosomes duplicate.
 Occurs when the cell is between cell division
 Interphase stages:
 G1: Cells grow to mature size
 S: DNA is copied
 G2: Cell prepares for division
 At the end of interphase, each chromosomes consists
of two genetically identical sister chromatids attached
together.
 Chromosomes are not yet visible under the
microscope; they are in a form called chromatin
Prophase I
 Most complex phase of meiosis and typically occupies over 90% of the
time required for meiotic cell division.
 Chromatin coils up so that individual chromosomes become visible
 A process called synapsis occurs, and homologous chromosomes, each
composed of two sister chromatids, come together as pairs.
 Resulting structure, consisting of four chromatids, is called a tetrad.
 Chromatids of homologous chromosomes exchange segments in a
process called crossing over:
 Rearranges genetic information, since homologues may be
different from each other.
 This genetic shuffling makes an important contribution to the
genetic variability resulting from sexual reproduction.
Prophase I
Prophase I
 As prophase I continues, the chromosomes condense
further as the nucleoli disappear.
 Spindle fiber forms
 Nuclear envelope breaks down
Metaphase I
 The chromosome tetrads are aligned on the metaphase
plate, midway between the two poles of the spindle.
 Each chromosome is condensed and thick, with its
sister chromatids still attached at their centromeres
 Spindle microtubules are attached at centromeres.
 In each tetrad, the homologous chromosomes are held
together at sites of crossing over.
 Within each tetrad, the spindle microtubules attached
to one of the homologous chromosomes from one pole
of the cell, and the microtubules attached to the other
homologous chrome come from the opposite pole
 Getting set up to separate the homologous chromsomes!
Metaphase I
Anaphase I
 Sister chromatids remain attached, however, the
tetrads split up .
 The sister chromatids move to opposite poles
 The cells are now containing half of the genetic
information from the original parent cell and are thus
considered HAPLOID!
Anaphase I
Telophase I and Cytokinesis
 Chromosomes arrive at the poles of the cell
 Each pole of the cell has a haploid chromosome set,
although each chromosome is still in duplicate form at
this point= each chromosome still consists of two
sister chromatids.
 Cytokinesis occurs along with telophase I and two
haploid daughter cells are formed.
Telophase I
Before Meiosis II…
 In some organisms, the chromosomes uncoil and the
nuclear envelope re-forms, and there is an interphase
before meiosis II begins.
 IN other species, daughter cells produced during the
first meiotic division immediately begin preparation
for the second meiotic division.
 In either case, NO chromosome duplication occurs
between telophase I and the onset of meiosis II.
Meiosis II
 In organisms having an interphase after meiosis I, the
chromosomes condense again and the nuclear
envelope breaks down during prophase II.
 In any case, meiosis II is essentially the same as
mitosis.
 The key difference is that meiosis II starts with a
haploid cell.
Prophase II
 A spindle forms and moves the chromosomes toward
the middle of the cell.
Metaphase II
 Chromosomes are aligned on the metaphase plate as
they are in mitosis, with the centromeres of the sister
chromatids pointing towards opposite poles.
Anaphase II
 centromeres of sister chromatids finally separate
 the sister chromatids of each pair, now individual
chromosomes, move toward opposite poles of the cell.
Telophase II
 Nuclei form at the cell poles, and cytokinesis occurs at
the same time.
 There are now four daughter cells, each with the
haploid number of (single) chromosomes.
Meiosis Animation
 http://www.sumanasinc.com/webcontent/animations
/content/meiosis.html
Mitosis vs. Meiosis
 Mitosis
 Provides growth, tissue repair, and asexual reproduction
 Produces daughter cells genetically identical to the
parent cell
 Involves one division of the nucleus, and is usually
accompanied by cytokinesis, producing two diploid
daughter cells.
 Meiosis
 Need for sexual reproduction
 Entails two nuclear and cytoplasmic divisions
 Yields four haploid daughter cells, with one member of
each homologous chromosome pair.
 Form tetrads; crossing over occurs.
Meiosis: Genetic Variation
 We’ve discussed how mutations lead to genetic variation.
 Also, The arrangement of homologous chromosomes pairs
at metaphase of meiosis I affects the resulting gametes.
 The orientation of homologous pairs in the center of the
cell is random; thus producing gametes with random
chromosomes
 For any species, the total number of combinations of
chromosomes that meiosis can package into gametes is 2n,
where n is the haploid number.
 For a human, 223, or about 8 million possible chromosome
combinations
 This means that each gamete you produce contains one of
roughly 8 million possible combinations of chromosomes
inherited from your mother and father.
Meiosis: Genetic Variation
 Possibility when a gamete from one individual unites
with a gamete from another individual in fertilization:
 In humans, the random fusion of a single sperm wit a
single ovum during fertilization will produce a zygote
with any of 64 trillion (8 million x 8 million)
combinations of chromosomes!
Meiosis: Genetic Variation
 Homologous chromosomes
 Bear two different kinds of genetic information for the
same characterisitic
 The key to what really makes gametes—and therefore
offspring--different
Meiosis: Genetic Variation
 Crossing Over:
 An exchange of corresponding segments between two
homologous chromosomes.
 Occurs during prophase I of meiosis.
 Chromosomes are a tetrad—four chromatids, with each
pair of sister chromatids joined at their centromeres.
 Each gene on each homologue is aligned precisely with
the corresponding gene on the other homologue
 Sites of crossing over appear as X-shaped regions; each is
called a chiasma.
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place where two homologous chromatids are attached to each
other.
 Can produce new combinations of genes= genetic
recombination!
Meiosis: Genetic Variation
 Crossing over:
 1. The DNA molecules of two nonsister chromatids—one maternal and
one paternal—break at the same place.
 2. Immediately, the two broken chromatids join together in a new way .
 In effect, the two homologous segments trade places, or cross over,
producing hybrid chromosomes with new combinations of maternal
and paternal genes.
 Called “recombinants”
 3. When the homologous chromosomes separate in anaphase I, each
contains a new segment originating form its homologues.
 4. Finally, in meiosis II, the sister chromatids separate, each going to a
different gamete.
**In meiosis in humans, an average of one to three crossover events occur
per chromosome pair.
Crossing Over
Meiosis: Genetic Variation
 In summary, there are three sources of genetic
variability, besides mutations, in sexually reproducing
organisms:
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1. crossing over during prophase I
2.independent orientation of chromosomes at metaphase 1
3.random fertilization
Karyotypes
 The term karyotype refers to the chromosome
complement of a cell or a whole organism.
 A karyotype is an ordered display of magnified images
of an individual’s chromosomes arranged in pairs,
starting with the longest.
 In particular, it shows the number, size, and shape of
the chromosomes as seen during metaphase of
mitosis.
 Chromosome numbers vary considerably among
organisms and may differ between closely related
species.
Karyotypes
 Karyotypes are prepared from the nuclei of cultured
white blood cells that are ‘frozen’ at the metaphase
stage of mitosis.
 Shows the chromosomes condensed and doubled
 A photograph of the chromosomes is then cut up and
the chromosomes are rearranged on a grid so that the
homologous pairs are placed together.
 Homologous pairs are identified by their general shape,
length, and the pattern of banding produced by a
special staining technique.
Karyotypes
 Male karyotype
 Has 44 autosomes, a single X chromosome, and a Y
chromosome (written as 44 + XY)
 Female karyotype
 Shows two X chromosomes (written as 44 + XX)
Karyotype- Normal
Karyotype- Down Syndrome
Down Syndrome
 Trisomy 21; named after John Langdon Down, who
characterized the syndrome in 1866
 47 chromosomes total; there are three number 21
chromsomes
 In most cases, a human embryo with an abnormal
number of chromosomes is spontaneously aborted
(miscarried) long before birth.
 Some chromosome abnormalities upset the genetic
balance less drastically, and individuals carrying them
can survive.
Down Syndrome
 Trisomy 21 is the most common chromosome number abnormality.
 Affects about one out of every 700 children born, and is the most
common serious birth defect in the US
 Symptoms include characteristic
 facial features, notably a round face, a skin fold at the inner corner of
the eye, a flattened nose bridge
 small, irregular teeth, as well as short stature
 heart defects, and susceptibility to respiratory infections, leukemia,
and Alzheimer’s disease.
 Exhibit varying degrees of mental retardation.
 Some live to middle age or beyond, and many are socially adept and
able to hold jobs.
 Most are sexually underdeveloped and sterile
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A few women have had children, however, half of their eggs will have the
extra chromosome 21, so there is a 50% chance that she will transmit the
syndrome to her child.
Down Syndrome
 Down Syndrome incidence:
 The incidence of Down Syndrome in the offspring of
normal parents increases remarkedly with the age of the
mother.
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Strikes less than 0.05% of children (fewer than one in 2,000)
born to women under age 30.
Risk climbs to 1.25% for mothers in their early 30s and is even
higher for older mothers.
Because of this relatively high risk, pregnant women over 35 are
candidates for fetal testing for trisomy 21 and other
chromosomal abnormalities.
Maternal age and incidence of
Down Syndrome
Errors in Meiosis
 Nondisjunction
 Members of a chromosome fail to separate.
 Can lead to an abnormal chromosome number in any
sexually reproducing diploid organism.
 For example, if there is nondisjunction affecting human
chromosome 21 during meiosis I, half the resulting
gametes will carry an extra chromosome 21.
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Then, if one of these gametes unites with a normal gamete,
trisomy 21 (Down Syndrome) will result.
Errors is Meiosis
 Two ways that nondisjunction can occur:
 1.a pair of homologous chromosomes does not separate
during Meiosis I.
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Two of the gametes produced will n + 1(abnormal), and two
will be n- 1 (abnormal).
 2. meiosis I is normal, but one pair of sister chromatids
fails to separate during meiosis II.
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Two of the resulting gametes are n + 1 and n-1 (abnormal) and
two will be n (normal)
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Abnormal gametes that get fertilized, will result in a zygote
with an extra chromosome.
Mitosis will then transmit the anomaly to all embryonic cells,
causing some syndrome linked to abnormal genes.
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Errors in Meiosis
 What causes nondisjunction?
 We do not yet know the answer, nor do we fully
understand why offspring with trisomy 21 are more likely
to by born as a woman ages.
 We do know, however, that meiosis begins in a woman’s
ovaries before she is born but is not completed until years
later, at the time of an ovulation.
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Because only one egg usually matures each month, a cell might
remain arrested in the mid-meiosis state for decades.
Some research points to an age-dependent error in one of the
checkpoints that coordinates the process of meiosis.
Errors in Meiosis
 Nondisjunction can also occur in sex chromosomes.
 For example, Klinefelter’s Syndrome :
 males have an extra X chromosome, making him XXY
 occurs approximately 1 out of every 2,000 live births
 Have male sex organs, but the testes are abnormally small
and the individual is sterile.
 Often includes breast enlargement and other female body
characteristics.
 Person is usually of normal intelligence
Errors in Meiosis
 Nondisjunction in sex chromosomes:
 Turner Syndrome
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Females who are lacking an X chromosome= XO, the O
indicates the absence of a second chromosome
Characteristic appearance include short stature, and often a
web of skin extending between the neck and the shoulders
Sterile; their sex organs do not fully mature at adolescence
If left untreated, girls will have poorly developed breasts and
other secondary sexual characteristics.
Normal intelligence
*Sole known case where having 45 chromosomes is not fatal.
Errors in Meiosis
 Nondisjunction in sex chromosomes:
 Males with XYY and females with XXX are normal.
Errors in Meiosis
 Abnormalities in chromosome structure:
 Breakage of a chromosome can lead to a variety of
rearrangements affecting the genes of that chromosome:
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1. deletion: if a fragment of a chromosome is lost.
 Usually cause serious physical and mental problems.
 Deletion of chromosome 5 causes cri du chat syndrome: child
is mentally retarded, has a small head with unusual facial
features, and has a cry that sounds like the mewing of a
distressed cats. Usually die in infancy or early childhood.
2.duplication: if a fragment from one chromosome joins to a
sister chromatid or homologous chromosome.
3.inversion: if a fragment reattaches to the original
chromosome but in the reverse direction.
 Less likely than deletions or duplications to produce harmful
effects, because all genes are still present in normal number
Errors in Meiosis
 Abnormalities in chromosome structure:
 Translocation
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The attachment of a chromosomal fragment to a
nonhomologous chromosome.
May or may not be harmful
For example, chromosomal translocation in a somatic cell in
the bone marrow is associated with chronic myelogenous
leukemia (CML), which is the most common type of leukemia,
the cancer that affects the cells that give rise to white blood cells
(leukocytes)
 Part of chromosome 22 has switched with a small fragment of
chromosome 9. The chromosome with the cancer-causing gene
is called the “Philadelphia chromosome”, after the city where it
was discovered.
Errors in Meiosis
Gregor Mendel
Gregor Mendel
 Background:
 Deduced the fundamental principles of genetics by
breeding garden peas.
 Known as the “Father of Genetics”
 Was a monk that lived and worked in an abbey in
Austria.
 In a paper in 1866, Mendel correctly argued that parents
pass on to their offspring discrete heritable factors.
 In his paper, he stressed that the heritable factors (today
called genes) retain their individuality generation after
generation.
Gregor Mendel
 Experiments:
 Chose to study garden peas because he was familiar with
them from his rural upbringing, they were easy to grow,
and they came in many readily distinguishable varieties.
 Also, he was able to exercise strict control over pea plant
matings.
 Due to their anatomical nature (petals of pea flower
almost completely enclose the stamen and carpel), pea
plants usually self-fertilize in nature.
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That is, sperm-carrying pollen grains released from the stamens
land on the egg-containing carpel of the same flower.
He used a small bag to cover the flower to ensure selffertilization.
Gregor Mendel
 Experiments (continued)
 He was also able to ensure cross-fertilization
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Fertilization of one plant by pollen from a different plant.
 Through his methods, Mendel could always be sure of
the parentage of new plants
 He chose seven characteristics, that occur in two distinct
forms ,to study:
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Flower color (purple, white)
Flower position (axial, terminal)
Seed color (yellow, green)
Seed shape (round, wrinkled)
Pod shape (inflated, constricted)
Pod color (green, yellow)
Stem length (tall, dwarf)
Gregor Mendel
 Experiments (continued):
 Mendel worked with his plants until he was sure he had true-
breeding varieties.
 For instance, he identified a purple-flowered variety that, when
self-fertilized, produced offspring that all had purple flowers.
 He then asked, What offspring would result if plants with purple
flowers and plants with white flowers were cross-fertilized?
 The offspring of two different varieties are called hybrids, and the
cross-fertilization itself is referred to as a hybridization, or simply
a cross.
 The true-breeding parental plants are called the P generation ( P
for parental).
 The offspring of the P generation are called the F1 generation (F
for filial, the Latin word “son”)
 When the F1 self-fertilize or fertilize with each other, their
offspring are called the F2 generation.
Gregor Mendel
 Mendel performed many experiments in which he tracked the inheritance of
characteristics that occur in two forms, such as flower color.
 Monohybrid cross:
 When you’re looking only at one trait (ex, flower color)
 Mendel performed a monohybrid cross between a pea plant with purple flowers
and one with white flowers.
 The F1 offspring all had purple flowers (not a lighter purple, has predicted by
a “blending” hypothesis.)
 Was the white gene lost?
 By mating the F1 plants, Mendel found the answer to be NO!
 Out of 929 F2 plants, Mendel found that 705 (about ¾) had purple flowers
and 224 (about ¼) had white flowers, a ratio of about three plants with
purple flowers to one with white flowers in the F2 generation (3:1)
 The heritable factor for white flowers did not disappear in the F1 plants,
but the purple-flower factor was the only one affecting the F1 flower color.
 The F1 plants must have carried two factors for the flower-color
characteristic, one for purple and one for white.
Gregor Mendel
Gregor Mendel
 Mendel observed these same patterns of inheritance for
six other pea plant characteristics.
 From these results, he developed four hypotheses, which
we will describe using modern terminology (such as
“gene” instead of “heritable factor”):
Gregor Mendel
 Hypothesis 1:
 There are alternative forms of genes that account for
variations in inherited characteristics.
 For example, the gene for flower color in pea plants exists
in two forms, one for purple and the other for white.
 The alternative versions of a gene are now called alleles.
Gregor Mendel
 Hypothesis 2:
 For each characteristic, an organism inherits two alleles,
one from each parent. These alleles may be the same ore
different.
 An organism that has two identical alleles for a gene is
said to be homozygous for that gene (and is called a
homozygote).
 An organism that has two different alleles for a gene is
said to be heterozygous for that gene (and is called a
heterozygote)
Gregor Mendel
 Hypothesis 3:
 If the two alleles of an inherited pair differ, then one
determines the organism’s appearance is called the
dominant allele; the other has no noticeable effect on
the organism’s appearance and is called the recessive
allele.
 We use upper-case letters to represent dominant alleles
and lowercase letters to represent recessive alleles.
Gregor Mendel
 Hypothesis 4:
 A sperm or egg carries only one allele for each inherited
trait because allele pairs separate (segregate) from each
other during the production of gametes.
 This statement is now known as the law of segregation.
 When sperm and egg unite at fertilization, each
contributes its allele, restoring the paired condition in the
offspring.
Gregor Mendel
 The right hand side of the diagram on the previous slide explains the
results of Mendel’s experiment.
 In this example, P represents the dominant allele (for purple
flowers) and p represents the recessive allele (for white flowers).
 At the top, you see the alleles carried by the parental plants both
were true-breeding.
 Mendel proposed that one parent had two alleles for purple flowers
(PP) and the other had two alleles for white flowers (pp)
 Consistent with hypothesis 4, the gametes of Mendel’s parental
plants each carried one allele; thus, the parental gametes in Figure
9.3B are either P or p.
 As a result of fertilization, the F1 hybrids each inherited one allele for
purple flowers and one for white.
 Hypothesis 3 explains why all of the F1 hybrids are (Pp) had purple
flowers; the dominant P allele has its full effect in the heterozygote,
while the recessive p allele had no effect on flower color.
Gregor Mendel
 The right hand side of the diagram on the previous
slide explains the results of Mendel’s experiment
(continued…)
 Mendel’s hypotheses also explain the 3:1 ratio in the F2
generation; because the F1 hybrids are Pp, they make
gametes P and p in equal numbers.
 You can see the possible gamete combinations using a
Punnett square.
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Used to make predictions about the possible phenotypes and
genotypes of offspring.
Gregor Mendel
 Genotype vs. Phenotype
 Phenotype
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For example, purple or white flowers.
Term used to describe an organism’s appearance, or expressed
physical traits.
Phenotypic ratio- ratio of the phenotypes of the offspring.
 Example, the ratio of purple flowers to white flowers is 3:1
 Genotype
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For example, PP, Pp, or pp.
Term used to describe an organism’s genetic makeup.
Genotypic ratio- ratio of the genotypes of the offspring.
 Example, the 1:2:1 is the ratio of PP, Pp, pp
Homologous Chromosomes
Remember, two homologous chromosomes may bear either the
same alleles or different ones. Thus, we see the connection
between Mendel’s laws and homologous chromosomes: Alleles
(alternate forms) of a gene reside at the same locus on
homologous chromosomes.
Test Cross
 Test Cross:
 a mating between an individual of unknown genotype
and a homozygous recessive individual.
 Mendel used testcrosses to determine whether he had
true-breeding varieties of plants.
 Continues to be an important tool of geneticists for
determining genotypes.
Not so simple…
 Although Mendel’s laws are valid for all sexually
reproducing organisms, they stop short of explaining
some patterns of genetic inheritance.
 In fact, for most sexually reproducing organisms, cases
where Mendel’s laws can strictly account for the
pattern of inheritance are relatively rare.
 More often, the inheritance patterns are more
complex…
Incomplete dominance
 Complete dominance
 The dominant allele had the same phenotypic whether
present in one or two copies.
 Incomplete dominance
 The F1 hybrids have an appearance in between the
phenotypes of the two parental varieties.
Incomplete Dominance
 For example, red snapdragons crossed with white
snapdragons produced hybrid flowers in the F1 with
pink flowers.
 This third phenotype results from flowers of the
heterozygote having less pigment than the red
homozygotes.
 The F2 generation would then have a 1:2:1 ratio of red,
pink, white.
Incomplete dominance
 In humans, an example involves the condition
hypercholestrolemia, dangerously high levels of
cholesterol in the blood, caused by a recessive allele(h),
due to a lack of LDL receptors.
 hh individuals have about 5 times the normal amount of
blood cholesterol and may have heart attacks as early as
age 2.
 Normal individuals are HH.
 Heterozygotes have blood cholesterol about twice
normal.
 Usually prone to atherosclerosis, the blockage of arteries
by cholesterol buildup in artery walls, and they may have
heart attacks from blocked heart arteries by their mid-30s.
Codominance
 Both alleles are expressed in the heterozygous
individual
 Different from incomplete dominance, which is the
expression of one intermediate trait
 Can be seen in blood type
Codominance
 The ABO blood group phenotype in humans involves
three alleles of a single gene.
 These three alleles, in various combinations, produce
four phenotypes: a person’s blood group may be either
O, A, B, or AB.
 These letters refer to two carbohydrates, designated A
and B, that may be found on the surface of red blood
cells.
 A person’s red blood cells may have carbohydrate A
(type A blood), carbohydrate B (type B), both (type AB),
or neither (type O).
Codominance
 Matching compatible blood groups is critical for safe
blood transfusions.
 If a donor’s blood cells have carbohydrate (A or B) that
is foreign to the recipient, then the recipient’s immune
system produces blood proteins called antibodies that
bind specifically to the foreign carbohydrates and cause
donor blood cells to clump together, potentially killing
the recipient.
Codominance
 Four blood groups result from various combinations of
the three different alleles, symbolized as IA, IB, and i.
 Each person inherits one of these alleles from each
parent.
 IA and IB are dominant to the i allele, but are
codominant to each other = both alleles are expressed
in the heterozygote IAIB , who have the blood type AB
 There are six possible genotypes:
 IAIA and IAi= A
 IBIB and IBi= B
 IAIB= AB
 ii = O
Dihybrid cross
 Dihybrid cross:
 Results from a mating of parental varieties differing in
two characteristics.
 For example: Mendel crossed homozygous round yellow
seeds (RRYY) with plants having wrinkled green seeds
(rryy).
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
All of the offspring in the F1 generation had round yellow
seeds; which raised the question: are the two characteristics
transmitted from parent to offspring as a package, or was
each characteristic inherited independently of the other?
The question was answered when Mendel allowed
fertilization to occur among the F1 plants the offspring
supported the idea that the two seed characteristics
segregated independently.
 Offspring had nine different genotypes, and four different
phenotypes with 9:3:3:1 ratio.
Dihybrid Cross
 Mendel’s results supported the hypothesis that each
pair of alleles segregates independently of the other pairs
of alleles during gamete formation Mendel’s Law of
Independent Assortment
Pleiotropy
 Most genes influence multiple characteristics, a
property called pleiotropy.
 An example of pleiotropy in humans is sickle-cell
disease
 Refer to p. 168
Polygenic Inheritance
 Polygenic inheritance is the additive effects of two or
more genes on a single phenotypic characteristic
 Examples include human skin color and height
 Different then pleiotropy, in which a single gene affects
several characteristics
Environmental affects
 Many characteristics result from a combination of heredity
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
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and environment.
For example, in humans nutrition influences height,
exercise alters build, sun-tanning darkens the skin, and
experience improves performance on intelligence tests.
It is becoming clear that human phenotypes—such as risk of
heart disease and cancer and susceptibility to alcoholism
and schizophrenia—are influenced by both genes and
environment.
Simply spending time with identical twins will convince
anyone that environment, and not just genes, affect a
person’s traits.
However, only genetic influences are inherited…cannot pass
on environmental influences to future generations!
Chromosome Theory of
Inheritance
 The chromosome theory states that genes occupy
specific loci (positions) on chromosomes and it is the
chromosomes that undergo segregation and
independent assortment during meiosis.
 Thus, it is the behavior of chromosomes during meiosis
and fertilization that accounts for inheritance patterns.
Linked genes
 Genes located close together on the same chromosome
tend to be inherited together and are called linked
genes.
 Linked genes generally do not follow Mendel’s law of
independent assortment.
 Refer to figure 9.19 in book
Crossing over
 As we saw in meiosis, crossing over between
homologous chromosomes produces new
combinations of alleles in gametes
 Forms recombinant gametes
Sex Chromosomes
 Sex chromosomes, designated X and Y, determine an
individual’s sex.
 XX individuals ar e female, and XY individuals are male
 Human males and females both have 44 autosomes
(nonsex chromosomes)
 As a result of chromosome segregation during meiosis,
each gamete contains one sex chromosome and a
haploid set of autosomes (22).
 All eggs contain a single X chromosome; sperm either
contain an X or Y
 An offspring’s sex is determined by whether the sperm
cell that fertilizes the egg bears an X or Y
Sex Chromosomes
 The genetic basis of sex determination in humans is
not yet completely understood, but one gene on the Y
chromosome plays a crucial role.
 This gene is called SRY (sex-determing region of Y) and
triggers testis development.
 In the absence of SRY, an individual develops ovaries
rather than testes.
 SRY codes for proteins that regulate other genes on the Y
chromosome, which in turn produce proteins necessary
for testis development.
Sex-linked genes
 Besides bearing genes that determine sex, the sex
chromosomes also contain genes for characteristics
unrelated to femaleness and maleness.
 Sex-linked genes are genes located on either sex
chromosomes, although in humans the term has
historically referred specifically to a gene on the X
chromosome. ***Be careful not to confuse the term sexlinked gene with the term linked genes!!!***
 Refer to figure 9.23A-D on p. 176
Pedigrees
 Pedigree is a family tree used to study how particular
human traits are inherited.
 It is analyzed using logic and the Mendelian laws
Goals of Pedigree Analysis
 1. Determine the mode of inheritance: dominant, or
recessive, sex-linked or autosomal
 2. Determine the probability of an affected offspring
for a given cross.
Basic symbols
More Symbols
Dominant or Recessive?
 Is it a dominant pedigree or a recessive pedigree?
 1. If two affected people have an unaffected child, it must be a
dominant pedigree: D is the dominant mutant allele and d is the
recessive wild type allele. Both parents are Dd and the normal child is
dd.
 2. If two unaffected people have an affected child, it is a recessive
pedigree: R is the dominant wild type allele and r is the recessive
mutant allele. Both parents are Rr and the affected child is rr.
 3. If every affected person has an affected parent it is a dominant
pedigree.
Dominant Autosomal Pedigree
I
2
1
II
1
2
3
4
5
6
III
1
2
3
4
5
6
7
8
9
10
Assigning Genotypes for
Dominant Pedigrees
 1. All unaffected are dd.
 2. Affected children of an affected parent and an unaffected parent
must be heterozygous Dd, because they inherited a d allele from the
unaffected parent.
 3. The affected parents of an unaffected child must be heterozygotes
Dd, since they both passed a d allele to their child. (also called
carriers)
 4. If both parents are heterozygous Dd x Dd, their affected offspring
have a 2/3 chance of being Dd and a 1/3 chance of being DD.
Recessive Autosomal Pedigree
Assigning Genotypes for
Recessive Pedigrees
 1. all affected are rr.
 2. If an affected person (rr) mates with an unaffected person, any
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unaffected offspring must be Rr heterozygotes, because they got a r
allele from their affected parent.
3. If two unaffected mate and have an affected child, both parents must
be Rr heterozygotes.
4. Recessive outsider rule: outsiders are those whose parents are
unknown. In a recessive autosomal pedigree, unaffected outsiders are
assumed to be RR, homozygous normal.
5. Children of RR x Rr have a 1/2 chance of being RR and a 1/2 chance of
being Rr. Note that any siblings who have an rr child must be Rr.
6. Unaffected children of Rr x Rr have a 2/3 chance of being Rr and a 1/3
chance of being RR.
Outsider Rules
 In any pedigree there are people whose parents are unknown. These
people are called “outsiders”, and we need to make some assumptions
about their genotypes.
 Sometimes the assumptions are proved wrong when the outsiders have
children. Also, a given problem might specify the genotype of an
outsider.
 Outsider rule for dominant pedigrees: affected outsiders are assumed
to be heterozygotes. (or carriers)
 Outsider rule for recessive pedigrees: unaffected (normal) outsiders
are assumed to be homozygotes.
 Both of these rules are derived from the observation that mutant alleles
are rare.
Autosomal Dominant
 All unaffected individuals are homozygous for the normal
recessive allele.
AutosomalDominant
Look for:
 Trait in every generation
 Once leaves the pedigree does not return
 Every person with the trait must have a parent with
the trait
 Males and females equally affected
Autosomal Dominant
Autosomal Dominant
Autosomal Recessive
 The recessive gene is located on 1 of the autosomes
 Letters used are lower case ie bb
 Unaffected parents (heterozygous) can produce affected offspring (if
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they get both recessive genes ie homozygous)
Inherited by both males and females
Can skip generations
If both parents have the trait then all offspring will also have the trait.
The parents are both homozygous.
E.g. cystic fibrosis, sickle cell anaemia, thalassemia
Autosomal Recessive
Look for:
 Skips in generation
 Unaffected parents can have affected children
 Affected person must be homozygous
 Males and females affected equally
Autosomal Recessive
AutosomalRecessive
Sex Linked Inheritance
 Genes are carried on the sex chromosomes (X or
Y)
 Sex-linked notation
 XBXB normal female
 XBXb carrier female
 XbXb affected female
 XBY normal male
 XbY affected male
Sex-linked dominant
 Mothers pass their X’s to both sons and daughters
 If the mother has an X- linked dominant trait and is
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homozygous (XAXA) all children will be affected
 If Mother heterozygous (XAXa) 50% chance of each child being
affected
Fathers pass their X to daughters only.
 Affected males pass to all daughters and none of their sons
 Genotype= XAY
Normal outsider rule for dominant pedigrees for females, but for sexlinked traits remember that males are hemizygous and express whichever
gene is on their X.
XD = dominant mutant allele
Xd = recessive normal allele
E.g. dwarfism, rickets, brown teeth enamel.
Sex-Linked Dominant
Look for:
 More males being affected
 Affected males passing onto all daughter
(dominant) and none of his sons
 Every affected person must have an affected
parent
Sex-Linked Recessive
 males get their X from their mother
 fathers pass their X to daughters only
 females express it only if they get a copy from both parents.
 Females can only inherit if the father is affected and mother is a
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carrier (hetero) or affected (homo)
expressed in males if present
 More males than females affected (males inherit X from
mother)
recessive in females
 An affected female will pass the trait to all her sons
 Daughters will be carriers if father is not affected
Outsider rule for recessives (only affects females in sex-linked situations):
normal outsiders are assumed to be homozygous.
Males cannot be carriers (only have 1 X so either affected or not)
Can skip generations
E.g. colour blindness, haemophilia, Duchene muscular dystrophy
Sex Linked Recessive
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Look for:
More males being affected
Affected female will pass onto all her sons
Affected male will pass to daughters who will be a
carrier (unless mother also affected)
 Unaffected father and carrier mother can produce
affected sons
Sex-linked recessive
Sex-linked recessive