HUMAN GENETICSChapter 12

REVIEW

Humans have 46 chromosomes 44 are autosomes

22 pairs of homologous chromosomes 2 are sex chromosomes: X and Y

Females have two X chromosomes Males have one X and one Y

KARYOTYPES: MALE AND FEMALE

SEX-LINKED TRAITS

Many genes not related to sex are on the X chromosome

They are called sex-linked traits Example: Drosophila (Fruit fly) eye

color

DROSOPHILA EYE COLOR

Red eyes ( XR ) is dominant White eyes ( Xr ) is recessive

XrYXRXR

XRXr

XRY

XR Xr

Y

XRXr

XR

Xr XRXR

XR

Y

XRY

XrXR XRY

XrY

XRXr

XR

Xr

Xr

YXRXr

XrXr XRY

XrY

XrY

R = red-eye alleler = white-eye allele

Most sex-linked human disorders are due to recessive alleles Examples: hemophilia,

red-green color blindness These are mostly seen in males Why???

HUMAN SEX-LINKED DISORDERS

Figure 9.23A

SEX – LINKED INHERITANCE

A male receives a single X-linked allele from his mother, and will have the disorder (XcY)

A female has to receive the allele from both parents to be affected (XcXc )

A female with the gene for the trait on one of her X chromosomes is called a carrier (XcX). A carrier may not know she carries the gene for the trait until

she has a son with the trait.

HEMOPHILIA

Hemophilia is a deadly disease in which the individual’s blood does not clot properly.

It is caused by a recessive gene on the X chromosome. (Xh)

Women can be XX, XXh (carrier), or XhXh (affected).

Men can be XY or XhY (affected).

PEDIGREES

A pedigree is a diagram that shows how a trait is inherited over several generations.

Circles represent women, squares are men

Solid (dark) is affected (with disease, etc.)

Empty (white) is not affected Half filled is a carrier (heterozygous)

A high incidence of hemophilia has plagued the royal families of Europe

Figure 9.23B

QueenVictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

Alexis

PROBLEMS

A female has the genotype XX and she marries a man who is color blind (XcY). What is the probability that they will have a child who is color blind?

A female who is not color blind, but whose father was color blind marries a color blind male. What is the probability that they will have a child who is colorblind?

HUMAN GENETIC DISORDERS

Sex-linked disorders Affected allele is usually on X chromosome

Autosomal disorders Can affect men and women equally

Chromosome number disorders Chromosomes don’t separate evenly in meiosis

SEX-LINKED DISORDERS

Colorblindness – Cannot distinguish between red & green

Hemophilia – Blood clotting disorder

Muscular Dystrophy – Muscles weaken and eventually break down.

AUTOSOMAL DISORDERS

Albinism no hair or skin pigment

Cystic Fibrosis mucus clogs lungs

Tay-Sachs disease breaks down nervous system

AUTOSOMAL DISORDERS

Sickle-cell anemia abnormal protein causes RBCs to sickle and clump

together PKU (Phenylketonuria)

Body can’t break down a certain amino acid so toxic substance accumulates

nerve damage, retardation

Huntington’s Disease Autosomal Dominant Weakness and deterioration of brain No symptoms appear until middle age A person might not know he or she had the gene until

after having children.

CHROMOSOME NUMBER DISORDERS

Turner’s Syndrome X0 (sterile females)

Klinefelter’s Syndrome XXY (mentally retarded, boy, underdeveloped female

characteristics) Down’s Syndrome

Trisomy 21