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