1

Inheritance 2

•  Pedigrees •  Rules of Probability •  Variations

–  Incomplete Dominance –  Co-dominance –  Epistasis –  Polygenic traits

•  Genes are on chromosomes

•  Sex linkage –  Pedigrees for sex-linked

traits 14 February 2020

Pedigrees

Key Male

Female

Affected male Affected female

Mating

Offspring, in birth order (first-born on left)

If one unaffected parent tests positive and the other tests negative for a recessive allele associated with a genetic disorder then � •  What is the probability that

their first child will have the disorder?

•  Let’s assume that the first

child does not have the disease. What is the probability that she is a carrier?

Rules of probability

•  �And� rule –  The probability of both of two independent events

is the product of the probabilities

•  �Or� rule: –  The probability of either of two independent and

mutually exclusive events is the sum of the probabilities

Phenotype and Genotype

Sperm

Eggs

P

PPP Pp

p

pPp pp

3 1

Genotype ratio: 1:2:1 Phenotype ratio: 3:1 (because purple is dominant to white).

Pp x Pp Dominant mutations:�

gain-of-function

Recessive mutations:

loss-of-function

2

Incomplete Dominance

Heterozygote is intermediate

Codominance Human Blood Groups Phenotype Genotype Type A IA IA or IA i Type B IB IB or IB i Type AB IA IB Type O ii

In type AB, IA IB are codominant.

The individual expresses both phenotypes.

Fig. 14-11

GenotypeRed blood cell

appearancePhenotype

(blood group)

IAIA or IA i A

BIBIB or IB i

IAIB AB

ii O

Epistasis: interactions among loci

Gene E determines whether dark pigment is deposited or not

Gene B determines color of pigment (Black or brown)

Black or Brown

pigment or no pigment

Yellow base color

B-E- Black bbE- Brown --ee Yellow

If no pigment is deposited (ee), it doesn�t matter what allele is present at the B locus

What phenotype ratio of offspring do you expect if you cross two heterozygous black labs (BbEe x BbEe)?

BE Be bE be BE BBEE BBEe BbEE BbEe

Be BBEe BBee BbEe Bbee

bE BbEE BbEe bbEE bbEe

be BbEe Bbee bbEe bbee

Epistasis example: In Morning Glories, two independent genes (A and B) are in the pathway that produces blue pigment. The recessive allele of each (a and b) produce nonfunctional enzymes so each recessive homozygote has white flowers.

•  If you cross two different pure-breeding white strains (AAbb x aaBB), what phenotype ratio will you observe in the offspring?

12

Blue pigment Enzyme A Enzyme B

Enzyme a Enzyme b

3

Multiple Genes

Eggs

Sperm

Phenotypes: Number of dark-skin alleles: 0 1 2 3 4 5 6

1/64 6/64 15/64 20/64 15/64 6/64 1/64

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8

AaBbCc AaBbCc

×

Human height is controlled by many genes

Genes are on Chromosomes

15

Nettie Stevens (1861�1912) X-Y chromosomes

Thomas Morgan (1866�1945) Linked genes

The mechanism of sex determination can vary

Male Female •  Mammals: XY XX •  Birds & Butterflies ZZ ZW •  Grasshoppers XX X •  etc

16

Data from T. H. Morgan, Sex-limited inheritance in Drosophila, Science 32:120–122 (1910).

Sex Linked Trait P

Generation

F1 Generation

Eggs

F2 Generation

Sperm

Sperm

Eggs

X X

X Y

w+

w+

w

w+ w+

w

w+

w+

w

w+

w+

w

w+

w

w+

w+

w

×