Lecture 7: Chromosomal Basis of inheritance

Chromosome theory of inheritance

Loci

Behavior of chromosomes during meiosis accounts for inheritance patterns

Loci

Genes location at specific positions on chromosomes

Karyotype

Display of chromosomes pairs of a cell arranged by shape and size

Prepared from somatic cells (diploid, 2n)

• 2 sets of chromosomes (1 from each parent)

Chromosomal basis of sex

Sex: Classification into group that shares a set of anatomical and physiological traits

Determined by inheritance of sex chromosomes in many species

Inheritance of x-linked genes

Morgan’s Experiment (1900s)

• X chromosomes- white-eyed mutant allele

• Female flies (xx) male (xy)

• Supported by theory of inheritance

Chromosomal basis of sex

X and Y chromosomes

Females (xx): Homogametic sex, X chromosome homologs

Males (xy): Heterogametic sex

• Short segments of x and y: Homologs

  • Allow x and y to pair during meiosis in the testes (like homologs)

Most genes on x not present in y and vise versa

Chromosomal systems of sex determination

Humans: XY

Numerals indicate # of autosomes in species

Grasshoppers: XO

Birds: ZW

Bees: Haplo-diploid system

Sex determination

Embyos: Generic until 2M

After, anatomical signs of sex begin to emerge

Gonads develop testes or ovaries (presence or absence of Y)

Sex determination region

Located on y in mammals

SRY (Sex-determining region of Y)

Required for testes to develop

WNTY for females

Sex linked genes

Located on x or y

1,100 x linked

78 y linked

Very few father-son transmission on y because low # of genes

X linked

Pattern of inheritance differ from that on autosomes

Fathers: pass to ALL daughters and NO sons

Mothers: pass to both

Color blindness (Red green)

X incantation in females

1 x in each cell in female mammals becomes inactive during early embryonic development

Female and male have same active copy of x linked gene

Tortie

Linkage affects inheritance

Linked Genes

• Located near each other on same chromosome

• Tend to inherit together

• Law of independent assortment deviation!!!

Genetic recomination

Offspring with combo of traits that differ from parents

Creates genetic variation

Unlinked and linked genes

Genetic recombination: Unlinked genes

From independent assortment

• Random orientation

• Located on different chromosomes (not linked)

• Offspring have combo of traits not found in parents

• Recombinant types or recombinents

• 50/50

Recombinant Frequency

= number of recombinants / total # of offspring

Genetic recombination: Linked

From crossing over in Prophase 1 of Meiosis

Breaks physical connection of alleles of genes on same chromosome

Always less than 50% recombinant frequency