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Bio 211
d15
Quiz on Mendelian genetics
 What are the two laws of Mendel
 What aspect of meiosis is responsible for these
outcomes?
Do Mendel’s rules always apply?
 Well, sometimes – as long as:
 Each trait is completely controlled by a single gene
 Only two possible alleles of each gene exist
 One allele is completely dominant to the other, recessive,
allele
Fly lab with Mendelian traits!
 Follow inheritance of several traits and see if they are Mendelian
 1. Choose individual traits and follow single trait – monohybrid crosses
for
 Wings: curved or dumpy
 Eyes: sepia, eyeless, or lobe
 2. Choose two that are Mendelian and follow dihybrid cross
 Wing/eye combo above
 Body and bristles combo
 Body: either black or ebony
 Bristles: either spineless or shaven
 3. Discuss non-Mendelian genetics
 Incomplete dominance
 Sex-linkage
 Polygenic inheritance
 4. Predict the outcome
 Of the wing cross if it were sex-linked
 Of the eye cross if it were incompletely dominant
Do Mendel’s rules always apply?
 Well, sometimes – as long as:
 Each trait is completely controlled by a single gene
 Only two possible alleles of each gene exist
 One allele is completely dominant to the other, recessive,
allele
 However:
 Many traits are influenced in more varied or subtle ways
So today… the million interesting exceptions







Sex linkage
Incomplete dominance
Multiple alleles
Co-dominance
Polygenic inheritance
Human genetic disorders!!!
And so much more…
Incomplete dominance
• phenotype of the heterozygotes is intermediate between the
phenotypes of the homozygotes
– In the genes studied by Mendel, one allele was dominant over the other,
which was recessive
– Some alleles, however, are only partially dominant over others
– When the heterozygous phenotype is intermediate between the two
homozygous phenotypes, the pattern of inheritance is called incomplete
dominance
– Human hair texture is influenced by a gene with two incompletely dominant
alleles, H1 and H2
•
•
•
•
•
A person with two copies of the H1 allele has curly hair
Homozygotes for the H2 allele have straight hair
Heterozygotes (H1H2) have wavy hair
In this case, can you tell genotype based on the phenotype?
If two wavy-haired people marry, their children could have any of the three hair types:
curly (H1H1), wavy (H1H2), or straight (H2H2)
• In what ratios?
– What is your hair phenotype? Predict your HH genotype.
Figure 10-13 Incomplete dominance
mother
H1H2
H1
H1H2
H2
H1
sperm
father
eggs
H1H1
H1H2
H1H2
H2H2
H2
Do all genes have only two alleles?
• A single gene may have multiple alleles
– An individual may have at most ___different gene alleles
– A species may have multiple alleles for a given
characteristic
– Human blood type is an example of multiple alleles of a
single gene
– Human blood group genes produce blood types A, B, AB,
and O
• There are ____ alleles in this system: A, B, and O
• So what are the genotypes and what are the phenotypes?
How does blood type work?
• Alleles A and B code for enzymes that add different sugar
molecules to the ends of glycoproteins that protrude
from red blood cells
• Allele O codes for a nonfunctional enzyme that doesn’t
add any sugar molecules
• Blood types A, B, AB, and O arise as a result of the
actions of these alleles
• Why does this cause a phenotype??
What are the genetic relationships
• People with AA or AO genotypes have blood type A
– A is dominant to O, which is recessive
• People with BB or BO genotypes have blood type B
– B is dominant to O, which is recessive
• People with OO genotypes have blood type O
– Just like the white-flowered plants are pp
• People with AB genotypes have blood type AB
– So what does that mean about A and B – are they dominant?
Recessive?
– They are co-dominant to each other!
What do these genes actually DO??
• Alleles A and B code for enzymes
– If A is present, enzyme A makes one kind of glycoprotein
– If B is present, a different glycoprotein is made
– In AB, both glycoproteins are present on the RBC surface
• O individuals lack the enzyme
– OO individuals have no enzyme, hence no A or B glycoproteins
The consequence of blood type
• We make antibodies to the glycoproteins we LACK
– People with type A blood make ___antibodies; people with
type B blood make ____ antibodies
– People with type O blood make ___ and _____antibodies; type
AB blood groups make _____ antibodies
– The antibodies cause red blood cells that bear foreign
glycoproteins to clump together and rupture
– The presence of such antibodies dictates that blood type must
be determined and matched carefully before a blood
transfusion is made
The consequence of blood type
• We make antibodies to the glycoproteins we LACK
– People with type A blood make B antibodies; people with type
B blood make A antibodies
– People with type O blood make both type A and type B
antibodies; type AB blood groups make no antibodies
– The antibodies cause red blood cells that bear foreign
glycoproteins to clump together and rupture
– The presence of such antibodies dictates that blood type must
be determined and matched carefully before a blood
transfusion is made
What is the best donor or recipient to be?
• Type O blood, lacking any sugars, is not attacked by
antibodies in A, B, or AB blood, so is transfused safely to all
– Type O blood is called the universal donor
– The A and B antibodies in type O blood become too dilute to
cause problems in the recipient of transfused type O blood
– Are O type good recipients?
– Because people with type O blood produce both A and B
antibodies, they can receive blood only from other type O donors
• Their antibodies would attack any donated blood cells bearing
A or B glycoproteins
• Best recipient would be ?
• AB (no antibodies to anything)
Table 10-1
Are all traits determined by just one gene?
• Many traits are influenced by several genes
– Some characteristics show a range of continuous
phenotypes instead of discrete, defined phenotypes
– Such as?
• height, skin color, and body build in humans
• These traits are influenced by interactions among two or
more genes through a process called polygenic inheritance
Polygenic inheritance
• Traits affected by polygenic inheritance are often strongly affected
by the environment, further blurring the differences among
phenotypes
• Human skin color is controlled by at least three genes, each with
pairs of incompletely dominant alleles
• According to research, human height is controlled by at least 180
genes
Figure 10-14 Polygenic inheritance of skin color in humans
Humans show a wide range of skin tones,
from almost white to very dark brown
Some genes affect more than one thing
• Some genes have multiple effects on phenotype
– Alleles that have multiple phenotypic effects are said to have
pleiotropy because they influence more than one phenotype
– The SRY gene on the Y chromosome in male humans encodes a
protein that activates other genes
• The SRY gene stimulates development of gonads into testes, which in
turn stimulates development of the prostate, seminal vesicles, penis,
and scrotum
Environmental effects
• The environment can profoundly affect phenotype
• Newborn Siamese cats demonstrate the effect of
environment on phenotype
– A Siamese cat has the genotype for dark fur all over its body
– However, the enzyme that produces the dark pigment is
inactive at temperatures above 93F (34C)
– When kittens are in the warmth of their mother’s uterus, the
enzyme is inactive and they are born with pale fur everywhere
– After birth, the ears, nose, paws, and tail become cooler than
the rest of the body, and dark pigment is produced there in the
pattern characteristic of the breed
Figure 10-16 Environmental influence on phenotype
Some genes are linked
• Genes on the same chromosome tend to be
inherited together
– Mendel’s law of independent assortment works only
for genes whose loci are on different pairs of
homologous chromosomes
– Alleles that are on the same chromosome do not line
up independently of one another on the metaphase
plate and are not separated at anaphase I
– Genes on the same chromosome tend to be inherited
together, a phenomenon called gene linkage
For example
• flower color and pollen in sweet peas are linked
–Meaning: these loci are on the same chromosome
–Purple flower color is dominant to red; long pollen
shape is dominant to round
–Let P = purple flowers and p = red flowers
–Let L = long pollen shape and l = round shape
– What are the expected gametes from parent PpLl,
where P is linked with L and p is linked with l ?
– Independent assortment would yield gametes in a
genetic proportion of 1/4 PL, 1/4 Pl, 1/4 pL, 1/4 pl
– Instead, the gametes are mostly PL and pl
Figure 10-17 Linked genes on homologous chromosomes in the sweet pea
flower-color gene
pollen-shape gene
purple
allele, P
long
allele, L
red
allele, p
round
allele, l
Can we separate normally co-inherited alleles?
• Crossing over creates new combinations of linked alleles
– Genes on the same chromosome do not always stay together
– Crossing over involves the exchange of DNA between
chromatids of paired homologous chromosomes in synapsis
– The farther apart two linked gene loci are on a chromosome,
the more likely crossing over is to occur between them
– Crossing over occurs so often between loci far apart on a
chromosome that they appear to assort independently
– When does the pairing and swapping happen?
– Crossing over, or genetic recombination, in prophase I of
meiosis creates new gene combinations
Figure 10-18a Duplicated chromosomes in prophase of meiosis I
flower-color gene
pollen-shape gene
sister
chromatids
purple allele, P
long allele, L
homologous
chromosomes
(duplicated)
at meiosis I
sister
chromatids
red allele, p
round allele, l
Duplicated chromosomes in prophase of meiosis I
Figure 10-18b Crossing over during prophase I
P
L
P
L
p
l
p
l
Crossing over during prophase I
Figure 10-18c Homologous chromosomes separate at anaphase I
recombined
chromatids
P
L
p
L
P
l
p
l
unchanged
chromatids
Homologous chromosomes separate at anaphase I
Figure 10-18d Unchanged and recombined chromosomes after meiosis II
recombined
chromosomes
P
L
p
L
P
l
p
l
unchanged
chromosomes
Unchanged and recombined chromosomes after
meiosis II
sex chromosomes!
 Animals have a set of sex chromosomes that dictate gender
 In mammals, females have two X chromosomes
 In mammals, males have an X chromosome and a
Y chromosome
• The Y chromosome is much smaller than the X chromosome
 A small section of the X and Y chromosomes is homologous,
allowing them to pair in prophase I and segregate during
meiosis
 The rest of the (non-sex) chromosomes occur in identical
pairs and are called autosomes
Figure 10-19 Human sex chromosomes
Y chromosome
X chromosome
Mammals – sex is determined by sperm
– For organisms in which males are XY and females
are XX, the sex chromosome carried by the sperm
determines the sex of the offspring
• During sperm formation, each sperm receives either the X or
the Y chromosome, along with a copy of each of the
autosomes
• Because the female has only X sex chromosomes, the
unfertilized egg must have an X chromosome
• If the egg is fertilized by a sperm with a Y chromosome, a
male results; if fertilized by an X-bearing sperm, a female is
produced
Figure 10-20 Sex determination in mammals
female parent
X1
X2
eggs
X1
X1
male parent
Y
X2
Xm
Xm
Xm
sperm
Xm
X2
female offspring
Y
X1
Y
X2
Y
male offspring
Y
Sex-linkage!
• Sex-linked genes are found only on the X or only on
the Y chromosome
– Genes carried on one sex chromosome, but not on the
other, are sex-linked
• In humans, the X chromosome is much larger than the Y and
carries over 1,000 genes
• In contrast, the human Y chromosome is smaller and carries only
78 genes
– During embryonic life, the action of the Y-linked gene SRY
sets in motion the entire male developmental pathway
• Under normal conditions, SRY causes the male gender to be
linked 100 percent to the Y chromosome
• Making being male a sex-linked trait
Most sex linked traits are on the X
– Few of the genes on the X chromosome have a
specific role in female reproduction
– Most of the genes on the X chromosome have no
counterpart on the Y chromosome
• Some genes found only on the X chromosome are
important to both sexes, such as genes for color vision,
blood clotting, and certain structural proteins in muscles
X linked genes – allele choice difference
between the sexes
– The X and the Y have very few genes in common
– Females (XX) can be homozygous or heterozygous for
a characteristic
– Males (XY) have only one copy of the genes on the
X or the Y
10.7 How Are Sex and Sex-Linked Traits
Inherited?
• Sex-linked genes are found only on the X or only
on the Y chromosome (continued)
– Because females have two X chromosomes, recessive
sex-linked genes on an X chromosome may or may not
be expressed
– Because males, with only one X chromosome, have no
second copy to mask recessive genes, they fully
express all the X-linked alleles they have, whether
those alleles are dominant or recessive
10.7 How Are Sex and Sex-Linked Traits
Inherited?
• Sex-linked genes are found only on the X or only
on the Y chromosome (continued)
– Red-green color blindness in humans is a sex-linked
trait
– Color blindness is caused by recessive alleles of either
of two genes located on the X chromosome
– The normal, dominant alleles of these genes (called C)
encode proteins that allow one set of eye cones to be
most sensitive to red light and another to be most
sensitive to green light
10.7 How Are Sex and Sex-Linked Traits
Inherited?
• Sex-linked genes are found only on the X or only
on the Y chromosome (continued)
– There are several defective recessive alleles of these
genes, called c
• The afflicted person cannot distinguish between red and
green
– A man can have the genotype CY or cY, which means
that he has a color-vision allele C or c on his
X chromosome and no corresponding gene on his
Y chromosome
10.7 How Are Sex and Sex-Linked Traits
Inherited?
• Sex-linked genes are found only on the X or only
on the Y chromosome (continued)
– He will have normal color vision if his X chromosome
bears the C allele, or be color-blind if his X
chromosome bears the c allele
– A woman may be CC, Cc, or cc because she has two X
chromosomes that each can carry an allele for the
trait, and will only be color-blind if her genotype is cc
10.7 How Are Sex and Sex-Linked Traits
Inherited?
• Sex-linked genes are found only on the X or only
on the Y chromosome (continued)
– A color-blind man (cY) will pass his defective allele
only to his daughters because only his daughters
inherit his X chromosome
– A heterozygous woman (Cc), although she has normal
color vision, will pass her defective allele to half her
sons, who will be color-blind
Figure 10-21 Sex-linked inheritance of red-green color deficiency
female parent
XC
Xc
eggs
Normal color vision
XC
Xc
The individual cannot distinguish red from green
XC
male parent
Y
Xc
XC
XC
sperm
XC
XC
female offspring
XC
Y
Xc
Y
Y
male offspring
Red-green color blindness
Expected children of a man with normal color vision
(CY), and a heterozygous woman (Cc)
Human Genetic Disorders
 Many human diseases are influenced by genetics
• Human geneticists trying to understand the relationship between
genetics and disease search medical, historical, and family
records to study past crosses
• Geneticists studying humans are proscribed from using breeding
techniques employed with plants and other animals
 Records of gene expression over several generations of a
family can be diagrammed (pedigree analysis)
 As a result, scientists now know the genes responsible for
sickle-cell anemia, hemophilia, muscular dystrophy,
Marfan syndrome, and cystic fibrosis
Single gene traits in humans
• Common traits eg. freckles, cleft chin, dimples
– Each trait controlled by a single gene with a recessive
and a dominant allele
• Some disorders are caused by defective alleles
– Defective allele may recessive or dominant
Recessive allele disorders
• New alleles produced by mutation usually code for
nonfunctional proteins
• Alleles coding for nonfunctional proteins are
recessive to those coding for functional ones
• The presence of one normal allele may generate
enough functional protein to enable heterozygotes
to be phenotypically indistinguishable from
homozygotes with two normal alleles
– Heterozygous individuals are carriers of a recessive genetic
trait (but otherwise have a normal phenotype)
– Recessive genes are more likely to occur in a homozygous
combination (expressing the defective phenotype) when
related individuals have children
• Close relatives are more likely than the general population to
each be heterozygous for a particular recessive allele and, so, are
more likely to produce the homozygous recessive phenotype
Eg. albinism
 Albinism results from a defect in melanin production
• Melanin is the dark pigment that colors skin cells
• Melanin is produced by the enzyme tyrosinase
• An allele known as TYR (for tyrosinase) encodes a
defective tyrosinase protein in skin cells, producing no
melanin and a condition called albinism
 Albino humans and animals are homozygotes
 Animals homozygous for TYR have no color in their skin, fur, or
eyes (the skin and hair appear white, and the eyes are pink)
Figure 10-23 Albinism
Human
Wallaby
Eg. sickle-cell anemia
•
caused by a defective allele for hemoglobin synthesis
–
–
–
–
Hemoglobin is oxygen-transporting protein in red blood cells
Mutant allele causes Hb molecules in blood cells to clump together
Red blood cells have a sickle (crescent) shape and easily break
Blood clots can form, leading to oxygen starvation of downstream tissues and paralysis
•
Considered a recessive disorder
–
People homozygous for the sickle-cell allele synthesize only defective hemoglobin and therefore
produce mostly sickled cells
Heterozygotes have about half normal and half abnormal hemoglobin, but usually have few
sickled cells and usually are not seriously affected
Usually only homozygotes for sickle-cell allele show symptoms
–
–
– About 5 to 25% of sub-Saharan Africans and 8% of African Americans are heterozygous for sickle-cell
anemia, but the allele is very rare in Caucasians
• Most common in Africa; uncommon in Causasian populations
– The large proportion of heterozygotes in Africa exists because heterozygotes have some resistance to the
parasite that causes malaria
– The rarity of heterozygotes in Caucasians corresponds with the rarity of malaria in northern climes, where
immunity (and therefore, heterozygosity) has no selective advantage
Figure 10-24 Sickle-cell anemia
Normal red blood cells
Sickled red blood cells
Dominant allele disorders
• can be transmitted to offspring if at least one parent suffers from
the disease and lives long enough to reproduce
– Dominant disease alleles also arise as new mutations in the DNA of eggs or
sperm of unaffected parents
• Various ways an allele can have dominance over the normal allele
– Some dominant alleles encode an abnormal protein
that interferes with the function of the normal protein
– Some dominant alleles encode proteins that carry out toxic reactions
– An allele may be dominant if it encodes a protein
that is overactive or is active at inappropriate times
and places
Eg. Huntington’s
 Huntington disease is caused by a defective protein
that kills cells in specific brain regions
• Huntington disease is a dominant disorder that causes
a slow, progressive deterioration of parts of the brain
– The disease results in a loss of coordination, flailing movements,
personality disturbances, and eventual death
• The disease becomes manifest in adulthood, ensuring its
maintenance in the population
• The gene encodes for a protein, called “huntingtin”, of
unknown function
– Mutant huntingtin seems both to interfere with the action
of normal huntingtin and to form large aggregates in nerve cells
that ultimately kill the cells
Sex linked disorders
• Some human genetic disorders are sex-linked
– The X chromosome contains many genes that have no
counterpart on the Y chromosome
– Males have only one X chromosome, so X-linked
diseases tend to be more commonly seen in men
Recessive sex-linked
– Sex-linked disorders caused by a recessive allele have a
unique pattern of inheritance
• A son receives his X chromosome from his mother and
passes it on only to his daughters, since the gene doesn’t
exist on his Y chromosome
• Sex-linked genes typically skip generations because the
affected male passes the trait to a phenotypically normal
carrier daughter, who in turn bears affected sons
• Several defective alleles for characteristics encoded on the X
chromosome are known, including red-green color blindness
and hemophilia
Eg. Hemophilia
• Hemophilia is caused by a recessive allele on the
X chromosome that results in a deficiency in one
of the proteins needed for blood clotting
– Hemophiliacs often have anemia owing to blood loss,
and bruise easily
– The hemophilia gene in Queen Victoria of England
was passed among the royal families of Europe
Figure 10-25 Hemophilia among the royal families of Europe
Edward
Duke of Kent
Albert
Prince
of SaxeCoburg-Gotha
Edward VII
King of
England
unaffected male
hemophiliac male
unaffected female
carrier female
Victoria
Princess of
Saxe-Coburg
Victoria
Queen
of England
Alexandra
of Denmark
Leopold
Duke
of Albany
Helen
Louis IV
Princess of
Grand Duke of
Waldeck-Pyrmont Hesse-Darmstadt
Alice
Princess
of Hesse
several
unaffected
children
Beatrice
Henry
Prince of
Battenburg
present British
royal family
(unaffected)
Victoria Elizabeth Alexandra
Mary
Tsarina
carrier
daughter
and
hemophiliac
grandson
Nicholas II Frederick Ernest Mary Irene
of Russia
Victoria
?
?
?
?
Olga
Tatiana
Maria
Anastasia
Alexander Alfonso Victoria Leopold Maurice
Queen
Albert
XII
of Spain
?
Alexis
Tsarevitch
Alfonso
Crown
Prince
Juan
Beatrice
?
died
Marie Jaime Gonzalo
in
infancy
Eg. aneuploidy
• Some human genetic disorders are caused by
abnormal numbers of chromosomes
– The incorrect separation of chromosomes or
chromatids in meiosis is known as nondisjunction
• Nondisjunction causes gametes to have too many and too
few chromosomes
– Most embryos that arise from fusion of gametes with
abnormal chromosome numbers spontaneously
abort, but some survive to birth and beyond
Figure 10-26 Nondisjunction during meiosis
Nondisjunction
during meiosis I
Normal meiosis
Nondisjunction
during meiosis II
Parent cell
Meiosis I
Meiosis II
n
n
n
n
n1
n1
n1
n1
n1
n1
n
n
Sex chromosome aneuploidy
• Some genetic disorders are caused by abnormal
numbers of sex chromosomes
– Nondisjunction of sex chromosomes in males or
females produces abnormal numbers of X and Y
chromosomes
– Nondisjunction of sex chromosomes in males produces
sperm with either no sex chromosomes (called “O”
sperm), or two sex chromosomes (sperm may be XX,
YY, or XY)
– Nondisjunction of sex chromosomes in females can
produce eggs that are O or XX eggs instead of eggs
with one X chromosome
How do you get sex chromosome
aneuploidy
• Normal gametes fuse with defective
sperm or eggs
– the zygotes have normal numbers of autosomes but
abnormal numbers of sex chromosomes
• Some common abnormalities
– XO, XXX, XXY, and XYY
• Some sex chromosome abnormalities allow
affected individuals to survive
– The genes on the X chromosome are so essential to
survival, any embryo without at least one X chromosome
spontaneously aborts very early in development
Eg. Turner’s syndrome
• Turner’s syndrome (XO) occurs in females with only one X
chromosome
– At puberty, hormone deficiencies prevent XO females from
menstruating or developing secondary sexual characteristics
– Hormone treatment promotes physical development, but
because affected women lack mature eggs, they remain infertile
– Additional symptoms include short stature, folds of skin around
the neck, and increased risk of cardiovascular disease, kidney
defects, and hearing loss
– Because they have only one X chromosome, women with
Turner’s syndrome are more susceptible to recessive disorders
such as red-green color blindness and hemophilia
Eg. trisomy X (XXX)
• Trisomy X (XXX) results in a fertile “normal”
woman with an extra X chromosome
– Most affected women show no abnormal symptoms
– There is an increased chance of learning disabilities
and a tendency toward tallness associated with
trisomy X
– By some unknown mechanism that prevents an extra
X chromosome from being included in their eggs,
women with trisomy X bear normal XX and XY children
Eg. Klinefelter syndrome (XXY)
• Men with Klinefelter syndrome (XXY) have an
extra X chromosome
– Most of these males show no symptoms, although
some may show mixed secondary sexual
characteristics, including partial breast development,
broadening of the hips, and small testes
– XXY men are often infertile because of low sperm
count, but are not impotent
– Diagnoses are made when an XXY male and his partner
seek medical help because of their inability to have
children
Eg. Jacob syndrome (XYY)
• Males with Jacob syndrome (XYY) have an extra Y
chromosome (XYY)
– Men with this malady have high levels of testosterone,
tend to develop severe acne, and may be exceptionally
tall
– Jacob syndrome occurs in about 1 in every 1,000 males
– Slightly increased likelihood of learning disabilities
Table 10-2
Autosome aneuploidy
• Some genetic disorders are caused by abnormal
numbers of autosomes
– Nondisjunction of autosomes can occur during meiosis
in the father or mother, resulting in eggs or sperm that
are missing an autosome or that have two copies of an
autosome
• Fusion of these gametes with a normal sperm or egg results
in a zygote with one or three copies of the affected
autosome
• Single-copy autosome embryos usually abort very early in
development
How do you get this
• Nondisjunction of autosomes
– can occur during meiosis in the father or mother,
resulting in eggs or sperm that are missing an
autosome or that have two copies of an autosome
• Embryos with three copies of an autosome
(trisomy) also usually spontaneously abort;
however, a small fraction of embryos with three
copies of chromosomes 13, 18, or 21 survive to
birth
• The frequency of nondisjunction increases with
the age of the parents
Eg. trisomy 21 (Down syndrome)
• In trisomy 21 (Down syndrome), afflicted
individuals have three copies of chromosome 21
– Occurs in about 1 out of every 800 births
– Down syndrome includes several distinctive physical
characteristics, including weak muscle tone, a small
mouth held partially open because it cannot
accommodate the tongue, and distinctively shaped
eyelids
– Down syndrome is also characterized by low resistance
to infectious diseases, heart malformations, and
varying degrees of mental retardation, often severe
Figure 10-27 Trisomy 21, or Down syndrome
Karyotype showing three
copies of chromosome 21
Girl with Down syndrome