Cytogenetics MD 3Y1-6 Group 3

1. Law of Independent Assortment
●
Mendel's 2nd
Law
● “Inheritance Law”
●
formulated after his 1st
principle
(law of segregation)
- alleles for a trait separate when
gametes
are formed
- allele pairs randomly unite at fertilization

2. ● What would happen if he studied plants that
differed in 2 traits?
● Would they be transmitted to the offspring
together, or
● Would one trait be transmitted independently of
the other?
● Performed dihybrid crosses (a cross b/w 2
parents that differ by 2 pairs of alleles; ex.
GGYY x ggyy) in plants that were true-breeding
for 2 traits
● Hybridization experiments done b/w
1856-1863 using garden peas (Pisum sativum)

3. ● First experiment: cross pollinated pure bred
round, yellow seed producing pea with pure bred
wrinkled, round seed producing pea.
● Cross pollinated a plant w/
green pod colour (G) & yellow seed colour (Y)
and a plant w/
yellow pod colour (g) & green seed colour (y)
● Dominant: green pod colour (GG)
yellow seed colour (YY)
● Recessive: yellow pod colour (gg)
green seed colour (yy)
● Resulting offspring/F1 generation:
all heterozygous for green pod colour and yellow
seeds (GgYy)

4. ● F1 plants:
– Genotype: GgYy
– Phenotype: green pod colour &
yellow seed colour
● both dominant traits

5. ● Allowed all of the F1 plants to self-pollinate and
referred to the resulting offspring as “F2
generation”

6. ● 9:3:3:1 phenotypic ratio
● F2 plants: 9 green pods & yellow seeds
3 green pods & green seeds
3 yellow pods & yellow seeds
1 yellow pod & green seeds
● 9 different genotypes
● 4 different
phenotypes

7. ● Mendel performed similar experiments focusing
on other traits including:
pod colour & pod shape
flower position & stem length
● phenotypic ratio of 9:3:3:1 present in each
● formulated his second principle, Law of
Independent Assortment from these findings:
allele pairs assort/separate independently
during the formation of gametes
● Biological selection for a particular gene in the
gene pair for one trait to be passed to offspring
has no relations with the selection of the gene
for any other trait
– true for genes that are not linked to each other

8. ● Independent assortment takes place during
meiotic metaphase I
● Produces a gamete with a mixture of the
organism's chromosomes
● Physical basis of independent assortment:
- random orientation of each bivalent
chromosome along the metaphase plate
with respect to the other bivalent
chromosomes
● Independent assortment + crossing over =
increased genetic diversity by producing novel
genetic combinations

9. ● Resulting chromosomes are randomly sorted
from all possible combinations of maternal &
paternal chromosomes
● Gametes are “assorted independently” b/c they
end up w/ a random mix instead of receiving a
pre-defined “set” from either parent
● Any possible combinations of gametes formed
from maternal & paternal chromosomes occur w/
equal frequency
● no. of possibilities for human gametes, w/ 23
pairs of chromosomes is 223 or 8,388,608
possible combinations
● This contributes to the genetic variability of
progeny (offspring)

10. ● Mendelian trait: trait controlled by a single locus
in an inheritance pattern
● Mutation in a single gene can cause a disease
that is inherited according to Mendel's laws
● Over 4000 human diseases caused by single
gene defects
● Disorders can be passed onto future generation
in several ways:
Autosomal dominance
Autosomal recessive
X-linked dominant
X-linked recessive
Y-linked

11. ● X-linked dominant
– Caused by utations in genes on X-
chromosome
– Males & females both affected w/ males
usually affected more severely
– Sons of man w/ disorder will be unaffected
since they receive father's chromosome;
daughters will inherit condition
– Woman w/ disorder has 50% chance of
having an affected fetus
– Some conditions like Rett syndrome,
incontinentia pigmenti type II & Aicardi
syndrome are often fatal in males in
utero or shortly after birth, so are more
predominantly seen in females

12. ● X-linked recessive
– Disorders also caused by mutations in
genes on X chromosomes
– Males more frequently affected than
females
– Sons of man w/ X-linked recessive
disorder will not be affected; daughters
will carry one copy of mutated gene
– Woman who is a carrier has 50% chance
of having sons who are affected; 50%
chance of having daughters who carry
one copy of mutated gene

13. ● X-linked recessive (cont.,)
– Serious diseases:
hemophilia A
Duchenne muscular dystrophy
Lesch-Nyhan syndrome
– Less serious conditions:
male pattern baldness
red-green colour blindness
– May manifest in females due to skewed
X-inactivation/ monosomy X (Turner
syndrome)

14. ● Y-linked
– Disorders caused by mutations on Y-
chromosome
– Since males inherit a Y chromosome from
their fathers, every son of an affected
father will be affected
– Since females only inherit X
chromosomes, female offspring are
never affected
– Since Y chromosomes are small &
contain very few genes, only a few Y-
linked disorders occur
– Symptoms usually include infertility (w/c
may be circumvented w/ help of fertility
treatments)

15. Disorder Prevalence (approximate)
X-linked
Duchenne muscular dystrophy 1 in
7000
Hemophilia 1 in
10,000
Sample Problem
A man w/ hemophilia marries a homozygous
normal
woman. Predict the genotypes and phenotypes
of
their children.
X-linked recessive trait: Hemophilia
XH X chromosome w/ normal dominant allele
Xh X chromosome w/ recessive hemophilia allele

16. Step 1. Determine the genotypes of the
parents.
Hemophiliac male Xh Y
Normal female XH XH
(Xh Y ) x (XH XH)
Step 2. Determine the gamete genotypes
produced by each parent.
(Xh Y) --> Xh , Y; --> XH
Step 3. Set up a Punnet square using the
gamete genotypes.
Xh Y
XH

17. Step 4. Combine the gamete genotypes of one
parent with those of the other parent to show all
possible offspring genotypes.
Step 5. State the genotype and phenotype ratios of
the offspring:
genotype: 1 (XH Xh): 1 (XH Y)
phenotype: 1 normal female : 1 normal male
Xh Y
XH XH Xh
XH Y

18. ● Modifier Genes
– Instead of masking the effects of another
gene, a gene can modify the expression
of the second gene
– ex. coat colour in mice
B: black coat
b: brown coat
D: full coat - modifier
gene
d: dilute (faded) coat - modifier
gene
Sample Problem:
Using a Punnet square, show the results
of mating 2 mice that are heterozygous
for both traits. Give the phenotypic ratio.

19. ● Modifier Genes
– Instead of masking the effects of another
gene, a gene can modify the expression
of another gene
– ex. coat colour in mice
B: black coat
b: brown coat
D: full coat - modifier
gene
d: dilute (faded) coat - modifier
gene
Sample Problem:
Using a Punnet square, show the results
of mating 2 mice that are heterozygous
for both traits. Give the phenotypic ratio.

20. BbDd x BbDd = BD, Bd, bD, bd; BD, Bd, bD, bd
9: full black coat (BD)
3: dilute black coat (B_dd)
3: full brown coat (bbD_)
1: dilute brown coat (bbdd)
BD Bd bD bd
BD BDBD BDBd BDbD BDbd
Bd BdBD BdBd BdbD Bdbd
bD bDBD bDBd bDbD bDbd
bd bdBD bdBd bdbD bdbd

21. ● Post Mendelian Genetics:
1. Co- Dominance
– 2 alleles are expressed at the same time
Eg. horses
RR: red
WW: white
RR x WW = RW (all are roan: red &
white)
2. Incomplete Dominance
– Strict dominance/recessiveness does not
apply
– Heterozygous individuals have an
intermediate phenotype
– Eg. Snapdragons
R: red
R': white

22. Sample Problem:
On planet XY, a plant exists where a red flower
allele (P) is incompletely dominant to the colour
blue (P'). Another pair of incompletely dominant
genes control pigment production where the
allele (I) gives no colour (white) and the allele (I')
gives full colour. Give the phenotypic ratio of the
offspring whose parents are heterozygous for
both traits. Use a Punnet square.
PP'II' x PP'II' = PI, PI', P'I, P'I' ; PI, PI', P'I,
P'I'

23. PPII': red faded – 2
PPI'I': red full – 1
P'PII': purple faded – 4
P'PI'I': purple full - 2
P'P'I'I: blue faded – 2
P'P'I'I': blue full – 1
PPII: (red) white – 1 white flowers = 4
PP'II: (purple) white – 2
P'P'II: (blue) white – 1
PI PI' P'I P'I'
PI PIPI PIPI' PIP'I PIP'I'
PI' PI'PI PI'PI' PI'P'I PI'P'I'
P'I P'PII P'IPI' P'IP'I P'IP'I'
P'I' P'I'PI P'I'PI' P'I'P'I P'I'P'I'