The Human Genome

Chapter 14

14.1: Human Heredity

• Humans have 46 chromosomes (23 from each parent)• 44 chromosomes are autosomes• 2 chromosomes are sex chromosomes• XX = female XY = male• Probability of female or male offspring?

• Karyotype: picture of chromosomes arranged by size.

Human Karyotype: Male of Female?

Pedigree• Biologists use pedigrees to study the inheritance

of traits from generation to generation. Often used to track diseases.

Blood Type• A and B are co-dominant• O is recessive• AB = Universal Receiver• O = Universal Donor• Phenotype: blood type

Genetic Diseases: Recessive • Phenylketonuria (PKU): Accumulation of

phenylalanine disrupts normal brain development; results in mental retardation

• Tay-sachs disease: Nerve damage from accumulation of chemical (gangliosides); fatal by age 4

• Albinism: cannot produce melanin (skin pigment), lack of color in hair, skin, and eyes.

Other animals can be albino too

Genetic Diseases: Dominant • Achondroplasia (dwarfism): shortened bone

growth• Huntington’s Disease: degeneration of neurons;

loss of motor control and cognition

Genetic Diseases: Co-dominant• Sickle cell anemia: Sickle/crescent shaped blood

cells; carry less oxygen and can become trapped in small blood vessels

• What are genes? Where are genes located?

• Remember, genes that are closer together are more likely to be inherited together. They are linked.

• Sex-linked genes are located on the X and Y chromosomes.

• All X-linked alleles are expressed in males, even if they are recessive. Why?

14.2 Human Chromosomes

Colorblind Test

Colorblind Test

Inheritance of Colorblindness

• Make a cross between a colorblind man (XbY) and a woman with normal vision (XBXB)

• Make a cross between a woman who carries the color blind allele (XBXb) and a man with normal vision (XBY).

• 1/10 males affected vs. 1/100 females

Other sex-linked disorders• Hemophilia: missing protein for blood clotting,

1/10000 males affected• Duchenne Muscular Dystrophy: weakening and

loss of skeletal muscle due to defective gene coding for muscle protein, in U.S. 1/3000 males affected

X-Chromosome InactivationCoat color in cats:  Black, Orange,  Calico

 

X B X B  = black, female

X O X O = orange, female

X B X O = calico, female

X B Y  = black, male

X O Y  = orange, male

CROSSES 

X B X B  (black female)      x     X O Y  (orange  male) 

This explains why all calico cats are female!

ALSO:  in any given cell, one of the X's becomes inactive, which randomizes the pattern of orange and black

NONDISJUNCTIONFailure of chromosomes to separate in

meiosis – results in an egg (sperm) with too

many or too few

• Down Syndrome (trisomy 21)

• Sex Chromosome Disorders

• Klinefelter Syndrome (XXY)—men are usually

infertile

• Turner Syndrome (XO)—woman are sterile

Nondisjunction during meiosis

Down Syndrome