Human Genetics

Genetics of sex

• Women & men are very different, but just a few genes create that difference

• In mammals = 2 sex chromosomes

– X & Y

– 2 X chromosomes = female: XX

– X & Y chromosome = male: XY

X Y

X X

Sex-Linked Traits

X chromosome

Y chromosome

Boy or Girl? The Y Chromosome “determines”

The Y chromosome of human males is only about one-third the size The Y chromosome of human males is only about one-third the size of the X chromosomeof the X chromosome

Sex-linked traits

• Sex chromosomes have other genes on them, too– especially the X chromosome

– hemophilia in humans• blood doesn’t clot

– Duchenne muscular dystrophy in humans• loss of muscle control

– red-green color blindness• see green & red as shades of grey

X Y

X X

• Sex-linked characteristics are found only on the X chromosome and not on the Y because Y is too small to carry these genes

XB XB – Normal Female

XB Xb – Carrier Female (do NOT show disorder)

Xb Xb – Diseased Female

XB Y - Normal Male

Xb Y - Diseased Male

Males only get 1 copy of the gene

• Sex-linked disorders are more common in males than females. Females must get both recessives genes to have the disorder while it only takes 1 for males

Sex-Linked Traits

• Carriers can pass the gene, but do not exhibit the symptoms of disorders

XXBBYY x x X XBBXXbb

NormalNormal CarrierCarrierMaleMale FemaleFemale

Sex-linked traits

XH Ymale / sperm

XH

Xhfe

mal

e / e

gg

s XHXH

XHXh

XHY

XhYXHXh

XH

Xh

XHY

Y

XH

XHXH XHY

XHXh XhY

2 normal parents,but mother is carrier

HH HhxXHY XHXh

Sex-Linked Traits

• Fathers= pass X-linked alleles to all daughters only (but not to sons)

• Mothers= pass X-linked alleles to both sons & daughters

Multiple Alleles in Human Blood

• There are 3 types of alleles (genes) in

human blood

IA

IB

IO or i

• A & B alleles are codominant• both A & B alleles are dominant over i allele

• We only have 2 of these three genes at a time, one from

each parent

• IA and IB are equally dominant and IO is recessive

Genotype Phenotype

IAIA IAi Type A

IBIB IBi Type B

IAIB Type AB

ii Type O

Co-dominance in Human Blood

IAIB IB

IA i

i

i i

IA i

IB

i

Co-dominanceType A x Type B

phenotypic ratio

= 1:1:1:1

1 type AB

1 type A

1 type B

1 type O

Many genes: one trait

• Polygenic inheritance– additive effects of many genes– humans

• skin color• height• weight• eye color• intelligence• behaviors

Albinism

albinoAfricans

melanin = universal brown color

Dominant ≠ most common allele

Polydactylydominant allele

• Because an allele is dominant does NOT mean…– it is better, or– it is more common

recessive allele far more common than dominant only 1 individual out of 500

has more than 5 fingers/toes so 499 out of 500 people are

homozygous recessive (aa)

Genetic Disorder

• Huntington’s disease (HD) is an inherited disorder caused by the degeneration of nerve cells in the brain.

• HD causes involuntary movements and loss of intellectual abilities (dementia).

Chromosomal Errors

• NONDISJUNCTION: the failure of chromosomes

to separate properly in meiosis. Gametes with

extra or too few chromosomes result.

Genetic Disorder

• Fragile X Syndrome: A disorder caused by a defective gene on the X-chromosome.

Genetic Disorder

• Symptoms of Fragile X Syndrome: – mental retardation, – Enlarged testes, – facial abnormalities in males – mild or no effects in females. It is the most common inherited cause of mental

retardation.

Genetic Disorder

• Cri-du-chat Syndrome is a rare genetic disorder due to a missing portion of chromosome # 5.

• Its name, meaning cat cry in French, is from the distinctive mewing sound made by infants with the disorder.

Genetic Disorders

• Cri-du-chat Syndrome is characterized by:– distinctive facial

features, – small head size,– low birth weight, – weak muscle tone, – a round face, – low set ears, – facial asymmetry– severe mental

retardation is typical

• Phenotype is controlled by both environment & genes

Color of Hydrangea flowers is influenced by soil pH

Human skin color is influenced by both genetics & environmental conditions

Environment effect on genes

Coat color in arctic fox influenced by heat sensitive alleles

Pedigree

• Pedigree charts are used to study the transmission of a hereditary condition.