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Unit 8: Genetics & Heredity
Unit 9: Human Genetic Disorders
Ch. 8: Heredity & Ch. 11: Human Genetics
heredity
Unit 8: Genetics & Heredity
• What is genetics?
– the study of heredity
• passing of traits from parents to offspring
Chromosomes in Cells
• Remember…
– body cells are diploid
• 2 of each chromosome
– 1 from mom & 1 from dad
– gametes (sperm & eggs)
are haploid
• 1 of each chromosome
– Why?
» So zygote gets right # of
chromosomes…
Genes
• Why is your combination of genes unique?
– Chance. Which sperm will fertilize which egg?
• get ½ your chromosomes from mom & ½ from dad
• meiosis (formation of gametes)
– crossing-over during prophase 1
– alignment of chromosomes during metaphase 1
Genes & Alleles
• What is a “gene”?
– section of chromosome that codes
for a specific protein
• & determines a specific trait (ex. hair
color, eye color, ear shape, etc.)
– genes are paired on
homologous chromosomes
(chromosomes that carry info
for same type of trait)
• different forms of genes for the
same trait are called “alleles”
– ex. brown eyes & blue eyes
Dominant & Recessive Alleles
• Each parent contributes 1 allele (form of gene) for
trait & can be dominant or recessive
– What is a dominant allele?
• allele that prevents expression of (“masks”/“hides”) recessive
trait
– What is a recessive allele?
• allele whose trait can be seen only when the organism is pure
(homozygous) for that trait
Dominant & Recessive Alleles
• How are alleles
represented?
– with letters
• usually the first letter
of the dominant trait
– If the same letter is
used for dominant &
recessive, how do we
know which allele is
which?
» CAPITAL = DOMINANT
» lowercase = recessive
Allele Combinations
• What does “homozygous” mean?
– both alleles are the same
• homozygous (pure) dominant (ex. AA)
• homozygous (pure) recessive (ex. aa)
• What does “heterozygous” mean?
– both alleles are different
• heterozygous (hybrid) (ex. Aa)
Genotype vs. Phenotype
• What is “genotype”?
– organism’s actual genetic
“code”/make-up (alleles)
• What does the genotype do?
– codes for protein that causes
trait (phenotype)
• How do we represent an
organism’s genotype?
– 2 letters (one for each allele)
• one from mom & one from dad
– ex. PP, Pp, pp
Genotype vs. Phenotype
• What is “phenotype”?
– the outward (physical) expression
of the genotype (trait we “see”)
• What actually causes the
“phenotype” (trait) we see?
– the protein that is produced (due
to the organism’s genotype
“code”/alleles)
• How do we represent an
organism’s phenotype?
– usually an adjective
• ex. purple, white, tall, short, etc.
Genotype is Expressed as a Phenotype
• Ex. Let P = purple & p = white
– homozygous (pure) dominant
• genotype PP
• phenotype = purple
– homozygous (pure) recessive
• genotype pp
• phenotype = white
– heterozygous (hybrid)
• genotype Pp
• phenotype = purple
– dominant trait “masks/hides”
recessive trait
PP
Pp
pp
Gregor Mendel
• Father of Genetics
– 1822-1884
• Wondered why
certain traits
disappear in one
generation, yet
reappear in the next
& tried to
determine how
traits were passed
from parent to
offspring.
Gregor Mendel
• studied garden pea plants with 7 different traits with clearly
different forms
– bred hybrids
– applied statistics
• Deduced that consistent ratios of traits in offspring
indicated that the plants transmitted distinct “units”
Mendel’s Experiments
• What happened when Mendel mated a pure purple
parent (PP) & a pure white parent (pp)?
– all F1 offspring:
• purple phenotype
• heterozygous (hybrid) genotype
– Pp
Mendel’s Experiments
• What happened when Mendel let the
heterozygous (hybrid) F1 offspring from his
first experiment self-pollinate?
– So… Pp x Pp (monohybrid cross)
• F2 offspring weren’t all purple… 3 purple : 1 white
– Always 3 :1 when both parents are hybrids
Parent
First filial
Second Filial
Crossed
two F1
plants to
get F2
Mendel’s Principle of Dominance
• What did Mendel notice
from his experiments?
– white x white always
produced white, but
purple x purple produced
~3/4 purple & ~1/4 white
• So, purple seemed to mask
white sometimes.
– …”dominant” trait
prevented expression of
“recessive” trait
» PUPRLE = dominant
» white = recessive
Mendel’s Law of Segregation
• during gamete
formation, each
gene for a trait
separates so that
each gamete
receives only 1 of
each gene
– happens during
meiosis I when
homologous
chromosomes line
up & separate
Test Cross
• Mendel determined two
genotypes resulting in
purple flowers with test
crosses
– Bred dominant phenotype
(unknown genotype) with
recessive phenotype
(“pure” homozygous since
shows recessive trait)
• If get all purple offspring 
parent = “pure”
homozygous PP
• If get ½ purple & ½ white
offspring  parent =
“hybrid” heterozygous Pp
Predicting Traits in Offspring
• What are Punnett Squares?
– a way to predict the results
of crosses (mating)
• letters outside represent
possible alleles in gametes of
each parent
– top = one parent & side = other
• letters inside boxes represent
possible allele combinations
(genotypes) in offspring
(& phenotypes)
Predicting Traits in Offspring
• Punnett Squares can also be used to determine
probability & ratios in possible offspring
BB
Bb
Making a Punnett Square
• Ex. Parents are Tt & tt genotypes…
– So… Tt x tt is our cross (mating)
Passing Traits to Offspring & Probability
• What is probability?
– chance an event will occur
• What is the chance of
getting heads? tails?
–½
• If you flip two coins, of
getting 2 heads? 2 tails?
– ½ x ½ = 1/4
• What is the chance of a
couple having a boy? a girl?
– 1/2
• of having five girls?
– ½ x ½ x ½ x ½ x ½ = 1/32
» or ( ½ )5 = 1/32
Passing Traits to Offspring & Ratios
• What is a
“genotypic ratio”?
– probable ratio of
genotypes (alleles)
in offspring of a
given cross
• Ex. If cross Pp & Pp
– 1PP : 2Pp : 1 pp
Passing Traits to Offspring & Ratios
• What is a
“phenotypic ratio”?
– probable ratio of
phenotypes (traits)
in offspring of a
given cross
– resulting from the
genotypes of the
offspring
• Ex. If cross Pp & Pp
• 3 purple : 1 white
Passing Traits to Offspring & Ratios
• What is an “expected ratio”?
– ratio we expect to get based
on probability (P. Square)
• What is an “observed ratio”?
– ratio we actually get
• Why would these be
different?
– fertilization is random
– some embryos die during
early stages
Mendel’s Principle of Independent Assortment
• Genes for different traits segregate independently during
gamete formation when they are located on different
chromosomes…
B
A
b
B
a
b
What if genes are on the same chromosome?
• called “linked”
• DO NOT sort independently
Genes on same
chromosome
meiosis
Genes on same
chromosome
Dihybrid Cross Animation
Dihybrid Cross
• involves study of inheritance patterns for organisms
differing in 2 traits (each w/ 2 forms).
– Mendel used dihybrid cross to determine if different
traits of pea plants, such as flower color & seed shape,
were inherited independently.
Dihybrid
Cross
2 traits
with 2
forms
Dihybrid Cross
• To figure out combination of genes in gametes for
two traits that are independently assorted use:
– “foil”
– (probability) tree diagram
• Ex. Parent w/ AaBb genotype will make the
following gametes:
– AB
– Ab
– aB
– ab
B
A
b
B
a
b
When both
parents are
hybrid for
both traits,
Always get
9:3:3:1
ratio
Dominant/Recessive is Not Always the
Method of Inheritance
• Traits are not always as clearly defined as the
7 pea plant traits Mendel studied.
– examples of non-dominant/recessive inheritance
• incomplete dominance
• codominance
• multiple alleles
• sex determination
• sex-linked traits
• polygenic inheritance
– Continuous variation
Incomplete Dominance
• No allele is dominant
over another
– results in 3 phenotypes –
“dominant” 1,
intermediate (mixed),
“dominant” 2.
• Genotypic & phenotypic
ratios same
– 1 CRCR : 2 CRCW : 1 CWCW
– 1 red : 2 pink : 1 white
– Ex. Pink four o’clock
flowers
Codominance
• What is meant by codominance?
– both alleles “expressed” equally
• Ex. Roan cow = mixture of both red & white hairs
Codominance
–Ex. human blood types exhibit codominance
(as well as multiple alleles)
• A & B are codominant & “expressed” equally
–IA = IB (codominant)
–i (recessive)
» So… (IA = IB ) > i
•How many possible genotypes are there?
•How many phenotypes?
•Can you spot the blood type that is the result
of codominance?
Multiple Alleles
• What is meant by
multiple alleles?
– more than 2 different
forms of an allele exist
• but individual has just 2
– 1 from mom & 1 from dad
– Ex. human blood types
• 3 alleles
– IA (A)
– IB (B)
– i (o)
•How many possible
genotypes are there?
•How many phenotypes?
•Can you spot the blood
type that is the result of
codominance?
antigen
antigen
antigens
No antigens
Multiple Alleles
• Agouti rabbits
– 4 alleles w/ dominance relationships
• Agouti is dominant to chinchilla; both are
dominant to Himalayan; all three are dominant
to albino… C > cch > ch > c
– agouti rabbit (wild type)
» Phenotype: brown, Genotype: CC, Ccch, Cch, Cc
– “Chinchilla” (mutant)
» Phenotype: silvery gray, Genotype: cchcch, cchch, cchc
– “Himalayan” (mutant)
» Phenotype: white w/ black, Genotype: chch or chc
– “Albino” (mutant)
» Phenotype: white, Genotype: cc
Sex Determination
• How many chromosomes do humans have (in
somatic cells)?
– 46… 23 pairs
• pairs 1 – 22 = autosomes (“body” chromosomes)
• 23rd pair determines gender = sex chromosomes
– XX = female
– XY = male
What is the
probability of
having a son?
A daughter?
Sex Determination
• Which parent’s chromosomes determines if
offspring will be a boy or girl???? Why?
– Dad’s
• if he gives X  girl
• If he gives Y  boy
– …mom always gives X
• so it can’t be her
What is the
probability of
having a son?
A daughter?
Sex-linked Inheritance
• X & Y chromosomes not fully homologous. Why?
– X is bigger & carries more genes
Sex-linked Inheritance
• How many alleles will a male have for
traits carried only on the X chromosome?
–ONE because only have one “X”
chromosome (Y doesn’t have allele)
• What is this called?
–X-linked or sex-linked
» Ex. eye color in fruit flies, hemophilia in humans,
colorblindness in humans
Sex-linked Inheritance
• X-linked (recessive) traits & disorders are
more common in males. Why???
– b/c female has XX, more likely she will have a
copy of dominant allele… males = XY… can only
get dominant allele on X (& only have 1 X)
• female can be XGXG, XGXg, XgXg
– normal, (normal) carrier, affected
• male can only be XGY or XgY
– normal or affected
Sex-linked Inheritance
• How do we make predictions made using
Punnett squares for sex-linked traits?
– Consider sex chromosome (X/Y) & allele for the
trait it carries (“exponent”) TOGETHER as a unit…
• ex. XG (= X w/ dominant allele), Xg (= X w/ recessive
allele), Y (= Y w/ NO allele)
Sex-linked Inheritance
• What if a female is heterozygous (XGXg)?
– she does not show the trait/have the disorder,
but is a carrier
• & can pass gene to offspring
• Can a male be a carrier?
– No, b/c only has one
X chromosome w/ allele…
so… either has it or doesn’t
XG
Y
XG
Xg
XG XG
XG Xg
XG Y
Xg Y
Sex-linked Inheritance
• Drosophila (fruit fly) eye color is sexlinked
–Let XR = red eye allele, Xr = white eye allele,
Y = no allele
• What are sex, phenotype, & genotype of each
offspring? Any carriers for white eye gene?
–female w/ red eyes = XRXR
–female w/ red eyes = XRXr
» carrier for white eye gene
–male w/ red eyes = XRY
–male w/ white eyes = XrY
XR
Xr
XR
Y
XR XR
XR Y
XR Xr
Xr Y
Polygenic Inheritance
• What is polygenic
inheritance?
–When many genes
affect a single trait
• shows range of
phenotypes from one
extreme to other
(continuous variation)
–Ex. in humans: hair
color, height, skin
color
Expression of Genes
• Genes can interact with one another to
control various other patterns of inheritance
– Most characteristics that make up individual’s
phenotype not inherited in Mendelian patterns
• Ex. Modifier genes affect eye color
– influence amount, intensity, & distribution of melanin
(color pigment) in eye cells
epistasis = phenomenon in which the
expression of one gene depends on the
presence of one or more 'modifier genes'
Expression of Genes
• Environment in which organism develops
is another factor that affects expression
– Probably due to how enzymes (proteins)
operate at different temperature
• Higher temps may “deactivate” enzyme &
prevent a reaction form occurring (therefore,
changing phenotype)
Expression of Genes
• Examples:
– Himalayan hare: temperature & fur color…
warm = white, cold = black
Expression of Genes
• Examples:
– tobacco: green dominant & albino recessive
• however color is also affected by environment
– If no sunlight, green color cannot be expressed due to
lack of chlorophyll production
» put in light  green will appear b/c chlorophyll
being produced
Unit 9: Human Genetic Disorders
• What causes genetic disorders?
– DNA mutation (usually recessive) or chromosome
abnormalities (in # or structure) that cause the
production of abnormal proteins
Human Genetic Disorders
• How can we group genetic disorders?
1. autosomal recessive disorders
(*most genetic disorders)
• allele is recessive & found on a chromosome from
pairs 1 – 22 (autosomes or body chromosomes)
– cystic fibrosis (CF), sickle-cell anemia, Tay-Sachs disease
2. autosomal dominant disorders
• allele is dominant & found on a chromosome from
pairs 1 – 22 (autosomes or body chromosomes)
– Huntington’s Disease
Human Genetic Disorders
3. sex-linked disorders
• allele (which is usually recessive) is found on the 23rd
pair of chromosomes (sex chromosomes)… Usually on
the X chromosome
– hemophilia, color blindness
4. chromosomal abnormality disorders
• result from errors in chromosome # or structure
– Down Syndrome (trisomy 21), Klinefelter’s Syndrome (XXY)
Autosomal Recessive Disorders
• What genotype(s) must a
person have to be affected?
– homozygous recessive (gg)
• cystic fibrosis
• sickle-cell anemia
• Tay-Sachs Disease
• Can someone be a carrier?
Why/why not?
– yes
• if heterozygous (Gg), person
carries gene, but isn’t affected
– due to having the “normal”
dominant gene
Autosomal Dominant Disorders
• What genotype(s) must a person
have to be affected?
– homozygous (GG) or heterozygous
(Gg) b/c allele is dominant
• Huntington’s Disease
• Can someone be a carrier?
Why/why not?
– No
• even if person is heterozygous (Gg),
will have disorder
– due to dominant “disease” gene
blocking “normal” recessive gene
Sex-linked Disorders
• Recall… hemophilia is X-linked & recessive
– What are the possible genotypes & phenotypes?
Can someone be a carrier?
• XHXH = normal female
• XHXh = carrier female (but not affected)
• XhXh = female w/ hemophilia
• XHY = normal male
• XhY = male w/ hemophilia
Sex-linked Disorders
–Why can’t a male be a carrier?
• b/c only has one X chromosome w/ allele…
so either has it or doesn’t
–Ex. mom = carrier & dad = normal:
• Make a Punnett square.
–genotypic ratio?
» 1 XHXH: 1 XHXh: 0 XhXh: 1 XHY: 1 XhY
–phenotypic ratio?
» 1 normal female: 1 carrier female (not affected) :
0 female w/ hemophilia: 1 normal male:
1 (affected) hemophiliac male
Sex-linked Disorders
• Recall… colorblindness is X-linked recessive
– What are the possible genotypes & phenotypes?
Can someone be a carrier?
• XCXC = normal female
• XCXc = carrier female (but not affected)
• XcXc = colorblind female
Ishihara
test for
• XCY = normal male
redgreen
c
color• X Y = colorblind male
blindness
Sex-linked Disorders
–In this Punnett square,
what are genotypes &
phenotypes of parents?
• father:
–genotype = XCY
–phenotype = normal
• mother:
–genotype = XCXc
–phenotype = carrier
(but she is not affected)
Chromosomal Abnormalities in Number
• What causes an
abnormal
number of
chromosomes?
– non-disjunction
• failure of paired
chromosomes
to separate
during meiosis 1
or meiosis 2
Disorders Due to Abnormal Chromosome #
• What is Down Syndrome (trisomy 21)?
– when person has 3 copies of chromosome # 21
• What causes Down Syndrome (trisomy 21)?
– non-disjunction
• failure of paired chromosomes to separate during
meiosis 1 or meiosis 2
Disorders Due to Abnormal Chromosome #
• What is Klinefelter’s Syndrome?
– a sex-chromosome disorder in which
males have extra copy of X chromosome
• XXY (or 47, XXY b/c 47 total chromosomes)
• What causes Klinefelter’s Syndrome?
– non-disjunction
• failure of paired chromosomes to separate
during meiosis 1 or meiosis 2
Chromosomal Abnormalities in Structure
• What is causes
structural
abnormalities in
chromosomes?
− pieces are
added,
deleted,
inverted, or
translocated
Detecting Abnormalities
• Karyotyping
–“picture of human
chromosomes”
• From blood sample
–Can detect extra
chromosomes or
chromosomal
abnormalities
(additions,
deletions,
inversions,
translocations)
Detecting Abnormalities
• Amniocentesis
– sample of fluid surrounding fetus
(karyotype then made)
• Can detect Down Syndrome
– 14th + week of pregnancy
• Chorionic villus biopsy
– sample of cells from chorion (part of structure by
which fetus linked to mother)
– 9th + week of pregnancy
Pedigree Charts
• A family tree (chart) of genetic history of
family over several generations
Square = male
Circle = female
Shaded =
studied trait
Marriage =
horizontal line
Offspring =
vertical line
Review & Animations
• Vocab interactive
– http://nortonbooks.com/college/biology/animations/ch10a02.htm
• Crosses
– http://www.sonefe.org/online-biyoloji-dersleri/grade-12/monohybrid-cross/
• Drag & drop genetics
– http://www.zerobio.com/drag_gr11/mono.htm
• Various
– http://www.abpischools.org.uk/page/modules/genome/dna4.cfm?coSiteNavig
ation_allTopic=1
• Genetic disorders
– http://www.humanillnesses.com/original/Gas-Hep/Genetic-Diseases.html