Biology 12

Unit 3: HeredityChromosomal Inheritance

The Source of Heredity• Chromosome Theory

– Genes are found on chromosomes– Paired chromosomes segregate during meiosis– Chromosomes (not genes) segregate independently– Each chromosome contains many genes

• Sex determination– In most animals the male of the species has a Y

chromosome.– Birds are a notable exception - many species of birds

has the XY condition as the female

Unexpected Results for Linked Genes

recombinants

Linkage ,Crossing Over & MappingA

A

B

B C

C

12

5

7

BA + AC BC

12 + 5 7

Therefore, A is not in the middle

AC + CB = AB

5 + 7 = 12

Therefore, C is in the middle!

Sex Linked Inheritance

♂♀ XN Y

XN

XN

♂♀ XN Y

XN

Xn

♂♀ Xn Y

XN

XN

♂♀ Xn Y

XN

Xn

♂♀ XN Y

Xn

Xn

♂♀ Xn Y

Xn

Xn

Pedigree Charts

Parents HeterozygousOffspring recessive

Parents HeterozygousOffspring recessive

Parents HeterozygousOffspring homozygous

dominant

Parents HeterozygousOffspring homozygous

dominant

Inconclusive

Inconclusive

CAN’T BE SEX LINKED RECESSIVE

CAN’T BE SEX LINKED RECESSIVE

CAN’T BE SEX LINKED RECESSIVE

IF SEX LINKED RECESSIVE SUSPECTED

CAN’T BE SEX LINKED RECESSIVE

MOM COULDBE A CARRIER

MOM COULDBE A CARRIER

1. What are everyone’s genotype?

2. What is the probability that III-3 and III-4 will produce another affected offspring?

I

II

III

IV

1 432

3 4

1

2

2

1

1 2