Chapter 5The inheritance of single-gene differences

• Inheritance patterns• Human pedigree analysis• Inheritance of organelle genes

The segregations of chromosomes at meiosisduring gamete production lead to preciseratios of genotypes and phenotypes in theprogeny. Segregation of the alles B = black,and b = brown in mice.

Inheritance patterns

The law of equal segregationfungal tetrads for segregation analysis

An A/a meiocyteundergoes meiosis,resulting in an equalnumber of A and aproducts.

Mendel, originator of thegene concept

pure lines,statistics,analysis of second and third generations

Phenotypes

Crossing

complete flowers

A monohybrid cross (self)

Test cross

Autosomal and Sex-linked Genes

1/2 X/Y1/2 X/XAll X

1/2 Y1/2 XGametes

Eggs

Sperm

The numerical equality of the two sexes isbased on equal segregation at meiosis.

Red-eyed and white-eyed Drosophila.The w (white) gene is located on the X chromosome of Drosophila.

Reciprocal crosses involving the X-linked w gene of Drosophila

White female Red male

Message

The results of reciprocal crosses involving sex linked genes(differential region of sex chromosomes) are different.

Different phenotypic ratios in male and female progeny arediagnostic for sex linkage.

Calico cat

Schematic diagram illustrating random X chromosome inactivation. The inactivation isbelieved to occur at about the time of implantation. A calico cat, with orange (O) andblack (o) alleles of a pigmentation gene on the X chromosome. The regions of differentcolor correspond to one or the other X chromosome being active.

X chromosome inactivation in mammals

Human Pedigree Analysis

Pedigree of a rare recessive phenotype (recessive allele a)

note:

individuals II/(1) andII(5) are assumed to beAA (rare allele frquency ina population);

the genotype of someindividuals can not bedetermined. Suchindividuals are indicatedby A/-.

Genetics and molecular biology of albinism

Pedigree of a rare dominant phenotype (dominant allele A). All genotypes can be deduced.

Pedigree of the human ability to taste the chemical phenylthiocarbamide PCP.These phenotypes constitute a genetic dimorphism, defined as the presence of twocommon forms of a character. In this case, none of the two alleles is rare, as is often thecase with alleles that case genetic disorders (see previous pedigree).

Human polymorphisms

Pedigree of a X-linked recessive allele.

Only males show thephenotype. In the nextgeneration, the sons do nothave that allele, however,the daughters carry therecessive allele.

Hemophilia in the British royal family

Pedigrees of an X-linked dominant disorders

Daughters of a male expressingan X-linked dominant phenotype,will all show the phenotype

Females with an X-linkeddominant phenotype usually areheterozygous, will all show thephenotype

Inheritance of Organelle GenesCytoplasmatic Inheritance

Mitochondrial and plastidic genes are often inherited maternally

Leaf variegation in Mirabilis jalapa