Sexual reproduction is the production of offspring that are genetically distinct from their parents. Most organisms have two sexual phenotypes—
The mechanism by which sex is specified is termed sex determination. Sex may be determined by differences in specific chromosomes, genotypes, or environment.
Sex chromosomes differ in number and appearance between males and females. The homogametic sex produces gametes that are all identical with regard to sex chromosomes; the heterogametic sex produces gametes that differ in their sex-
In the XX-
In some organisms, environmental factors determine sex.
In Drosophila melanogaster, sex is predicted by the X : A ratio, but is primarily determined by genes on the X chromosome.
In humans, sex is ultimately determined by the presence or absence of the SRY gene located on the Y chromosome.
Sex-
A female inherits X-
The fruit fly Drosophila melanogaster has a number of characteristics that make it an ideal model organism for genetic studies, including a short generation time, large numbers of progeny, small size, ease of rearing, and a small genome.
In placental mammals, one of the two X chromosomes in females normally becomes inactivated. Which X chromosome is inactivated is random and varies from cell to cell.
Y-
Dominance is an interaction between genes at the same locus (allelic genes). Dominance is complete when a heterozygote has the same phenotype as one homozygote. It is incomplete when the heterozygote has a phenotype intermediate between those of two parental homozygotes. In codominance, the heterozygote exhibits traits of both parental homozygotes.
Penetrance is the percentage of individuals having a particular genotype that exhibit the expected phenotype. Expressivity is the degree to which a character is expressed.
Lethal alleles cause the death of an individual possessing them, usually at an early stage of development, and may alter phenotypic ratios among the offspring of a cross.
The existence of multiple alleles refers to the presence of more than two alleles at a locus within a group of individuals. The presence of multiple alleles increases the number of genotypes and phenotypes that are possible.
Gene interaction refers to the interaction between genes at different loci to produce a single phenotype. An epistatic gene at one locus suppresses or masks the expression of hypostatic genes at other loci. Gene interaction frequently produces phenotypic ratios that are modifications of dihybrid ratios.
Sex-
In cytoplasmic inheritance, the genes for the characteristic are found in the organelles and are usually inherited from a single (typically maternal) parent. In genetic maternal effects, an offspring inherits genes from both parents, but the nuclear genes of the mother determine the offspring’s phenotype.
Genomic imprinting refers to characteristics encoded by genes whose expression is affected by the sex of the parent transmitting the genes. Epigenetic effects such as genomic imprinting are caused by alterations to DNA—
Phenotypes are often modified by environmental effects. A phenocopy is a phenotype produced by an environmental effect that mimics a phenotype produced by a genotype.
Continuous characteristics are those that exhibit a wide range of phenotypes; they are frequently produced by the combined effects of many genes and the environment.