Chromosomal basis of heredity
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Chromosomal Basis of Heredity. Studying Inheritance Patterns for Genes on Specific Chromosomes. Sex Determination. Sex Chromosomes : homologous chromosomes that differ in size and genetic composition between males and females. Autosomes. Autosome: any chromosome that is not a sex chromosome

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Chromosomal basis of heredity
Chromosomal Basis of Heredity

Studying Inheritance Patterns for Genes on Specific Chromosomes


Sex determination
Sex Determination

Sex Chromosomes: homologous chromosomes that differ in size and genetic composition between males and females


Autosomes
Autosomes

Autosome: any chromosome that is not a sex chromosome

Humans have 22 pairs of autosomes and 1 pair of sex chromosomes


Sex determination in mammals

X1

X2

Female Parent

EGGS

X1

X1

X1

X2

X2

X2

SPERM

Female Offspring

Male Parent

Xm

Xm

Xm

Xm

Y

Y

Y

Y

Male Offspring

Sex Determinationin Mammals

Expected Ratio½ female: ½ male


Sex linked genes eye color in fruit flies

R

r

XRY

XrY

XRXR

XRXr

R

Genotypes

Frequencies

Phenotypes

Frequencies

R = redr = white

Sex Linked Genes: Eye Color in Fruit Flies

Eggs of XR XrFemale

XR

XR

XR

Xr

Sperm ofXRY Male

Female

Female

Y

XR

Y

Xr

Male

Male

1

1

1

1

White male

Red female

Red female

Red male

25%

25%

25%

25%


Sex linked genes in humans

Females can be

homozygous or heterozygous for

traits on the

X chromosome

Males carry only

one copy of genes

on the X chromosome

H = no hemophilia h = hemophilia

Sex-linked Genes in Humans

GrampsXhY

Hemophilic Male Non-hemophilic

Female

(father is hemophilic)

XhY x XHXh

XH Xh

XHXh

XhXh

Xh

Y

XHY

XhY

Phenotypic Ratio of Offspring

¼ hemophilic males + ¼ non-hemophilic males

¼ hemophilic females + ¼ non-hemophilic females


Human chromosomal abnormalities

Normal Meiosis

Non-disjunction

in Meiosis I

Non-disjunction

in Meiosis II

Human Chromosomal Abnormalities

Arise by Non-disjunction =

Failure of homologues or chromatids

to separate during meiosis


Human autosomal abnormality
Human Autosomal Abnormality

How can Down Syndrome occur?

Eg. Egg with 2 copies of #21 (24 chromosomes)

+ Sperm with 1 copy of #21 (23 chromosomes)

= Embryo with 3 copies of #21 (47 chromosomes)


Incidence of down syndrome

400

300

200

100

0

10

20

30

40

50

Incidence of Down Syndrome

Number per 1000 Births

Age of Mother (years)


Karyotype for Down Syndrome

Physical Features

Eye fold

Palm Crease


Human sex chromosome abnormality
Human Sex Chromosome Abnormality

How can Turner Syndrome occur?

Eg. Egg with 0 copies of X (22 chromosomes) +Sperm with 1 copy of X (23 chromosomes)= Embryo with 1 copy of X (45 chromosomes)


Karyotype for Turner’s Syndrome

Non-functional Ovaries From Adult Female with Turner’s Syndrome

Normal uterus, tubes

and ovaries


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