Chromosomal Basis of HeredityChromosomal Basis of Heredity

Studying Inheritance Patterns for Studying Inheritance Patterns for Genes on Specific ChromosomesGenes on Specific Chromosomes

Sex DeterminationSex Determination

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

Human Human

ChromosomeChromosome

XX YY

SizeSize LargerLarger SmallerSmaller

Genetic Genetic

CompositionComposition

Multiple Multiple

genesgenes

unrelated unrelated

to genderto gender

Very few Very few genesgenes

TDF region TDF region determines determines maleness maleness

AutosomesAutosomes

Autosome:Autosome: any chromosome any chromosome that is not a sex chromosomethat is not a sex chromosome

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

XX11 XX22

Sex DeterminationSex Determinationin Mammalsin Mammals

EGGSEGGS

Male ParentMale ParentYYXXmm

SSPPEERRMM

Female OffspringFemale Offspring

Male OffspringMale Offspring

YY

XXmmXXmmXX11 XX22XXmm

YY YYXX11 XX22

Expected RatioExpected Ratio½ female: ½ male½ female: ½ male

XX11 XX22Female ParentFemale Parent

25%25%Red femaleRed female Red femaleRed female Red maleRed male

25%25% 25%25% 25%White male

FrequenciesFrequenciesPhenotypesPhenotypes

GenotypesGenotypes

FrequenciesFrequencies

Sex Linked Genes: Eye Color in Fruit FliesSex Linked Genes: Eye Color in Fruit Flies

Eggs of Eggs of XR Xr FemaleFemale

Sperm ofSperm ofXXRRY Male Y Male

1111

YXR

XRXrXRXR

YXr

XRXR XrYXRXr XRY

R r

R

Female Female

Male Male

11 11

R = redR = redr = whiter = white

Sex-linked Genes in HumansSex-linked Genes in Humans

Hemophilic Male Non-hemophilic Hemophilic Male Non-hemophilic Female Female (father is hemophilic)(father is hemophilic)

XXhh

YY

XXHH X Xhh

XXHHXXhh XXhhXXhh

XXHHYY XXhhYY

Phenotypic Ratio of OffspringPhenotypic Ratio of Offspring¼ hemophilic males + ¼ non-hemophilic males¼ hemophilic males + ¼ non-hemophilic males

¼ hemophilic females + ¼ non-hemophilic females¼ hemophilic females + ¼ non-hemophilic females

XXhhY x XY x XHHXXhhMales carry onlyMales carry onlyone copy of genes one copy of genes on the X chromosomeon the X chromosome

Females can beFemales can behomozygous or homozygous or heterozygous for heterozygous for traits on thetraits on the X chromosomeX chromosome

H = no hemophilia h = hemophiliaH = no hemophilia h = hemophiliaGrampsGramps

XXhhYY

Human Chromosomal AbnormalitiesHuman Chromosomal Abnormalities

Arise by Non-disjunction =Arise by Non-disjunction =Failure of homologues or chromatidsFailure of homologues or chromatidsto separate during meiosisto separate during meiosis

Normal MeiosisNormal Meiosis Non-disjunction Non-disjunction in Meiosis I in Meiosis I

Non-disjunction Non-disjunction in Meiosis II in Meiosis II

Human Autosomal AbnormalityHuman Autosomal Abnormality

How can Down Syndrome occur? How can Down Syndrome occur?

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

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

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

Down SyndromeDown Syndrome Trisomy 21Trisomy 21

Three copies of Three copies of chromosome 21chromosome 21

Incidence of Down Syndrome

1010 2020 3030 4040 505000

100100

200200

300300

400400

Age of Mother (years)Age of Mother (years)

Num

ber p

er 1

000

Birt

hsN

umbe

r per

100

0 B

irths

Karyotype for Down SyndromeKaryotype for Down Syndrome

Eye foldEye fold Palm CreasePalm Crease

Physical FeaturesPhysical Features

Human Sex Chromosome AbnormalityHuman Sex Chromosome Abnormality Turner SyndromeTurner Syndrome XOXO

One copy of XOne copy of X

No second sex No second sex chromosomechromosome

How can Turner Syndrome occur?How can Turner Syndrome occur?

Eg. Egg with 0 copies of XEg. Egg with 0 copies of X (22 chromosomes) (22 chromosomes)

+Sperm with 1 copy of X+Sperm with 1 copy of X (23 chromosomes) (23 chromosomes)

= Embryo with 1 copy of X= Embryo with 1 copy of X (45 chromosomes) (45 chromosomes)

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

Normal uterus, tubesNormal uterus, tubes and ovariesand ovaries

Karyotype for Karyotype for Turner’s SyndromeTurner’s Syndrome