More Genetics

Chromosomes and Human Genetics

A cool frog

Outline Key concepts Gene linkage Sex determination in human

Sex linkage

Sickle cell anemia

Errors in chromosome number

Key Concepts:Each gene has its own position in

sequence on a chromosome

Crossing over allows alleles in sequence to swap places

Allele recombination contributes to variations in phenotypes

Changes in chromosomes can give rise to genetic abnormalities or disorders

Gene linkageI. Linkage – if genes for 2 traits are located on

the same chromosome, they are linked. (genes on the same chromosome tend to be inherited together)

II. Drosophila (Fruit fly)P: Tan body & Long wings - TTLL

Black body & short wings – ttllF1: Tt Ll – all Tan body & Long wings

By mating F1 with each otherF2: ¾ Tan body & Long wings (T-L-)

¼ black body & short wings (ttll),Why?

Gene linkage

[Punnett Square]♂\♀ TL tlTL TTLL TtLltl TtLl ttll

some: Tan bodies & short wings (T-ll)black bodies & long wings (ttL-)

Crossing Over and Genetic Recombination

Homologous Chromosomes

Crossover

GeneticRecombination

Sex Determination in Humans

Females XX

Males XY

Human Embryo and Sex Organ Development

XY Embryo 8 Weeks old

Gene on Y chromosome governs development of testes

Early Human Embryo

Duct system in early human embryoDevelop into

male or female reproductive organs

Sex linkage The X chromosome has some genes that Y

chromosome does not have. Only one recessive gene can get expressed. A son receives his X chromosome from his mother and can pass it only to his daughters. Thus, sex-linked diseases often have a unique pattern – skip generations.

For example: red-green color blind & hemophiliaHemophilia is a disease in which the blood

does not clot normally. The disease is recessively inherited and the gene is carried on the X chromosome.

Sex linkageSuppose that a normal man marries a woman who is a carrier of the disease gene of hemophilia, what are the genotypic and phenotypic ratios of their children?XH = normal gene, Xh = hemophilia gene:

P: XHY X XHXh

F1: ♂\♀ XH Xh

XH XHXH XHXh

Y XHY XhY♂ : geno-ratio: ½ XHY, ½ XhY

pheno-ratio: ½ normal, ½ disease♀ : geno-ratio: ½ XHXH, ½XHXh

pheno-ratio: ½ normal, ½ carrier

Patterns of X-Linked Inheritance

X-Linked Recessive InheritanceHemophilia A

Color Blindness

Duchenne Muscular Dystrophy

Patterns of Autosomal Recessive Inheritance

Both Parentsare carriers

Sickle cell anemia

Sickle cell anemiaa recessive disease; one amino acid changes in hemoglobin molecule; Sickle-cell hemoglobin molecules tend to cluster together and block the capillary vessels.

normal SS, disease ss, carriers Ss. In Africa, 45% of certain population

have the genotype Ss. Because the heterozygotes (Ss) are resistant to the disease Malaria (a deadly disease).

Patterns of Autosomal Dominant Inheritance

Achondroplasia

Patterns of Autosomal Dominant Inheritance

One type of dwarfism in humans is caused by a singledominant gene; the condition is calle Achondroplastic dwarfism. Dwarf individuals are heterozygous, whilepersons who are homozygous recessive are of normalstature. The homozygous dominant individuals all diebefore birth. If two achondroplastic dwarfs marry and have a family,what fraction of their living offspring will also beachondroplastic dwarfs?

Dwarf Aa Normal aa AA die P: Aa X AaF1 genetupes: AA(die)Aa Aa aa 2/3 to be dwarf

Achondroplasia

Changes in Chromosome Number1. Chromosomes:

vary in nature from 2n = 2 to 2n = 1264human 2n = 46: ♂ 22 autosomes and XY

♀ 22 autosomes and XX

Barr bodies – every “extra” X produces a Barr body in

the nucleus (a ♀ has 1 Barr body; a ♂, none.)

2. Abnormal:

Klinefelter syndrome –22II + XXY, 2n = 47(♂), 1/1000

Turner syndrome – 22II + X, 2n = 45(♀), 1/5000

Speck syndrome – 22II + XYY, 2n = 47(♂), 1/1000

Trisomy: Down syndrome – 3 #21, 2n = 47(♂), 2n = 47(♀), 1/750

Trisomy X – 22II + XXX, 2n = 47(♀), 1/1000

Changes in theNumber of Autosomes

Down Syndrome

Changes in theNumber of Autosomes

Down Syndrome

Changes in theNumber of Chromosomes

Turner Syndrome

Changes in theNumber of

Chromosomes

Klinfelter Syndrome

Changes in theNumber of Chromosomes

Klinfelter Syndrome

Changes in theNumber of

Chromosomes

Speck Syndrome

Changes in theNumber of

Chromosomes

XXY Triploidy

Changes in theNumber of

Chromosomes

XXX Triploidy

Changes in theNumber of Chromosomes

Barr Bodies of XXXXY

Abnormal Chromosomes

In Conclusion

Genes are arranged in sequence on a chromosome

Humans have 23 pairs of homologous chromosomes

Females have two X chromosomes whereby males have an X and a Y

All other chromosomes are autosomes

In Conclusion

The parental chromosome number may be changed

Crossing over and random segregation adds to adaptive variation in traits among members of a population