Hour 5new

LECTURE 5

2.2.5 Lethal Genes2.2.6 Linked Genes

OBJECTIVES

At the end of the lesson, students should be able to :

• Explain dominant and recessive lethal allele.• Calculate genotypic and phenotypic ratios for

recessive lethal alleles (2:1) • Explain linked genes.• Show the effect of linked genes on the dihybrid test

cross (3:1) in the absence of crossing over.

• Show the effect of linked genes with crossing over on the dihybrid test cross ratio.

• Explain sex-linked genes.

Lethal Genes• lethal genes - genes that leads to the death.• Types of lethal genes.

- Dominant lethal allele - recessive lethal allele

Example for recessive lethal allele• E.g: gene for coat color of rodents (mice). • Wild mice have grey-colored fur (agouti) while mutants,

yellow. • A self cross between mice with yellow fur produces

offspring in the ratio of 2 yellow to 1 agouti.• These results suggest that the allele for yellow (Y) is

dominant to the allele for agouti (y). • So, although the Y allele is dominant for fur color, it is

recessive for the lethal characteristic .• Genotypic and phenotypic ratio is 2:1

If we cross two heterozygotes, expect a 3:1 ratio in progeny

But, in yellow x yellow cross, get:2 yellow : 1 nonyellow

Explanation:

Coat color of rodents

P : Yellow X Yellow Yy

Yy

G : Y y Y y

F1:

YY Yy yY yyYellow Yellow Yellow

Grey(Die)

Example of dominant lethal allele- Hungtinton’s disease- A degenerative disease of the nervous system- Has no obvious phenotypic effect until certain

age.- If Hh x hh , with H is dominant allele for

Hungtinton’s disease, 50% of child born inheriting the allele & the disorder.

Inheritance of Huntington’s allele

Linked Genes

Genes that are on the same

chromosome are linked. Such genes do not obey Mendel’s

laws because they do not undergo

independent assortment. They are inherited together unless

separated by crossing over during

prophase 1 of meiosis.

Linked genes in Drosophila

SEX DETERMINATION IN HUMANS

• Human sex is determined by a pair of sex chromosomes called X and Y.

• Because these chromosomes do not look alike, they are called heterosomes.

• All other chromosomes are called autosomes.

• Every human cell contains 23 pairs of chromosomes.

• Females have two large X chromosomes (XX), males have one X chromosome and one Y chromosome (XY).

• During meiosis, the sex chromosomes pair up and segregate into the daughter cells.

• Males are heterogametic sex because they produce different sperm: approximately 50% contain an X chromosome and 50% have a Y chromosome.

• Females produce homogametic sex because all of their egg contain an X chromosome.

Sex Linked Genes

• Human have 22 pairs of autosomes and a pair of sex chromosome.

• Genes carried on the sex chromosomes are said to be sex-linked.

• Human females have two X chromosomes , meaning they have two sex-linked alleles.

• In males, the Y chromosome is smaller and cannot mirror all the genes found on the X chromosome, so males have only one sex-linked allele.

• This is why males suffer from the effects of X-linked genetic diseases more often than females.

• There are no known Y-linked traits, probably because the Y chromosome carries so few genes.

XcXc

Color blind X XCY

Normal

Xc Xc XC Y

XCXc XCXc XcY XcY

Parent

gamete

progeny

100% carrier 100% color blind

Transmission of sex-linked genes