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The incidence of X-linked recessive conditions in females is the square of that in males: for example, if 1 in 20 males in a human population are red–green color blind, then 1 in 400 females in the population are expected to be color-blind (1 / 20)*(1 / 20).
A simple example to illustrate genotype as distinct from phenotype is the flower colour in pea plants (see Gregor Mendel). There are three available genotypes, PP (homozygous dominant), Pp (heterozygous), and pp (homozygous recessive). All three have different genotypes but the first two have the same phenotype (purple) as distinct from the ...
•List of human protein-coding genes page 2 covers genes EPHA1–MTMR3 •List of human protein-coding genes page 3 covers genes MTMR4–SLC17A7 •List of human protein-coding genes page 4 covers genes SLC17A8–ZZZ3 NB: Each list page contains 5000 human protein-coding genes, sorted alphanumerically by the HGNC-approved gene symbol.
By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and other consequences that may arise from expression of deleterious recessive traits resulting from incestuous sexual relationships and consanguinity. Animals avoid inbreeding only rarely. [2]
Mendelian traits behave according to the model of monogenic or simple gene inheritance in which one gene corresponds to one trait. Discrete traits (as opposed to continuously varying traits such as height) with simple Mendelian inheritance patterns are relatively rare in nature, and many of the clearest examples in humans cause disorders.
In a dominant-recessive inheritance, an average of 25% are homozygous with the dominant trait, 50% are heterozygous showing the dominant trait in the phenotype (genetic carriers), 25% are homozygous with the recessive trait and therefore express the recessive trait in the phenotype. The genotypic ratio is 1: 2 : 1, and the phenotypic ratio is 3: 1.
Autosomal dominant A 50/50 chance of inheritance. Sickle-cell disease is inherited in the autosomal recessive pattern. When both parents have sickle-cell trait (carrier), a child has a 25% chance of sickle-cell disease (red icon), 25% do not carry any sickle-cell alleles (blue icon), and 50% have the heterozygous (carrier) condition. [1]
The pattern of inheritance in which a single recessive allele is inherited but is still expressed is known as pseudodominance. This mainly occurs with sex-linked genes (i.e., those on the sex chromosomes). The homogametic sex (females in humans) receive two of each sex chromosome and therefore need to be homozygous to show a recessive trait.