HEREDITY: INHERITANCE

AND VARIATION

UNIT I - MODULE 2

When Gender Matters

How is sex/gender determined and inherited?

FemaleXX

X X

MaleXY

X Y

FemaleXX

FemaleXX

MaleXY

MaleXY

KEY CONCEPT:You will observe in the Figure, all egg

cells receive an X chromosome; while half of the sperm cells receive X chromosomes and the other half receive Y chromosomes.

If an egg is fertilized by a sperm with a Y chromosome, as shown in the Figure, the offspring is male. When an egg is fertilized by a sperm carrying an X chromosome, the offspring is female. Note that there is a 50 percent chance of having a male or female offspring. The greater the number of offspring, the greater is the chance of getting the expected 1:1 ratio of male and female.

Have you seen a man with hairy ears?What about a woman with hairy ears?

That is an example of a Y-Linked trait called Hypertrichosis pinnae auris, a

genetic disorder in humans that causes hairy ears.

Since the trait is found in the Y chromosome, then ONLY MALES can

have the trait. A father who has the condition will pass it on to all his sons and they, in turn, will pass it on to their

own sons.

INTRODUCTION

There are three kinds of sex-related inheritance:

1. sex-linked genes

2. sex-limited traits

3. sex- influenced traits

Sex-Linked Genes Genes located on the X chromosomes

are called X-linked genes. Genes on the Y chromosomes are

called Y-linked genes. An example of an X-linked trait in

humans is hemophilia. A person suffering from hemophilia could die from loss of blood even from a small wound because the blood either clots very slowly or does not clot at all. Another example of an X-linked trait is color blindness.

SEX - LINKED GENES The X chromosome with the gene for color blindness is

represented as XC, while the one without is represented as X.

GENOTYPES AND PHENOTYPES OF COLOR BLINDNESS IN HUMANS

GENOTYPE PHENOTYPE1. XX Normal Female2. XXC Normal Female, carrier of the gene

3. XCXC Color-blind female4. XY Normal male5. XCY Color-blind male

When Gender Matters

Solve problems related to sex-linked traits

1. Activity Sheet

2. Pen

1. Read the given problem: A. Color-blindness is a recessive, sex-linked

disorder in humans. A color-blind man has a child with a woman who is a carrier of the disorder.

KEY: X = normal vision Xc = color-blindness

2. Illustrate using a Punnett square the probability of having children who will have normal vision and children who will be color-blind.

1. What is the genotype of the male?

2. What is the genotype of the female?

3. What is the chance that the child will be color-blind?

4. What is the chance that a daughter will be color-blind?

5. What is the chance that a son will be color-blind?

When Gender Matters

X

XC

XXC

(Normal Female, carrier of gene)

XY(Normal Male)

XCXC

(Color-blind Female)

XCY(Color-blind

Male)

Color-blind Man

XC Y

Wom

an, c

arrie

r of

Dis

orde

r

1. What is the genotype of the male? XCY

2. What is the genotype of the female? XXC

3. What is the chance that the child will be color-blind? 50%

4. What is the chance that a daughter will be color-blind? 25%

5. What is the chance that a son will be color-blind? 25%

Sex - Linked traits are inherited through the X chromosomes.

Males have only one X chromosome. Thus, if they inherit the affected X, they will have the disorder.

Females have two X chromosomes. Therefore, they can inherit/carry the trait without being affected if it acts in a recessive manner.

A man and his wife both have normal color vision but a son has red-green color blindness, a sex-linked recessive trait.

The man sues his wife for divorce on grounds of infidelity. Can Genetics prove evidence supporting his case?

Color blindness is a recessive trait. The onlyway to have a child born that way is if both parents carry the gene. Assuming neither parent is color blind, both could still be carrying the gene and not know it.

So, if the father is tested and does not have the gene at all, he has a case. But he may well have the gene as a recessive trait. In this case, he is "half of the reason."

A man and his wife both have normal color vision but a daughter has red-green color blindness, a sex-linked recessive trait.

The man sues his wife for divorce on grounds of infidelity. Can Genetics prove evidence supporting his case?

X

X

XX(Normal Female)

XY(Normal Male)

XX(Normal Female)

XY(Normal Male)

HUSBAND

X Y

WIF

E

W

W

RW(ROAN)

WW(WHITE)

RW

(ROAN)WW

(WHITE)

ROAN MALE

R W

WH

ITE

FE

MA

LE

YES

The daughter displays the sex-linked recessive which means she got a recessive from both parents. Since the man does not display the recessive, he does not carry it, so the daughter has a different father.

1. What is sex-limited trait?

2. Give examples of sex-limited traits in animals.

Thank You

Mr. Christopher A. LunaScience 9 Teacher

GFMNHS -MAINJuly 10, 2014