The Laws of Probability in Genetics | Education - Seattle PI
Dominant and recessive inheritance are useful concepts when it comes to predicting the probability of an individual inheriting certain phenotypes, especially genetic . Keratin proteins link together to form strong fibers that strengthen hair. Then, it is completely down to chance what we inherit from them. Below is a table of dominant and recessive traits shown in humans. For example, the recessive genetic disease That is, the dominance relationships of any.
At heterozygous gene loci, the two alleles interact to produce the phenotype. Complete dominance[ edit ] In complete dominance, the effect of one allele in a heterozygous genotype completely masks the effect of the other.
The allele that masks the other is said to be dominant to the latter, and the allele that is masked is said to be recessive to the former. A classic example of dominance is the inheritance of seed shape pea shape in peas. Peas may be round associated with allele R or wrinkled associated with allele r. In this case, three combinations of alleles genotypes are possible: RR and rr are homozygous and Rr is heterozygous.
The RR individuals have round peas and the rr individuals have wrinkled peas. In Rr individuals the R allele masks the presence of the r allele, so these individuals also have round peas. Thus, allele R is completely dominant to allele r, and allele r is recessive to allele R. Incomplete dominance[ edit ] This Punnett square illustrates incomplete dominance. In this example, the red petal trait associated with the R allele recombines with the white petal trait of the r allele.
The plant incompletely expresses the dominant trait R causing plants with the Rr genotype to express flowers with less red pigment resulting in pink flowers. The colors are not blended together, the dominant trait is just expressed less strongly. Incomplete dominance also called partial dominance, semi-dominance or intermediate inheritance occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes.
For example, the snapdragon flower color is homozygous for either red or white.
When the red homozygous flower is paired with the white homozygous flower, the result yields a pink snapdragon flower. The pink snapdragon is the result of incomplete dominance.
A similar type of incomplete dominance is found in the four o'clock plant wherein pink color is produced when true-bred parents of white and red flowers are crossed. In quantitative geneticswhere phenotypes are measured and treated numerically, if a heterozygote's phenotype is exactly between numerically that of the two homozygotes, the phenotype is said to exhibit no dominance at all, i.
When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1: This diagram shows co-dominance. In this example a white bull WW mates with a red cow RRand their offspring exhibit co-dominance expressing both white and red hairs.
Co-dominance occurs when the contributions of both alleles are visible in the phenotype. For example, in the ABO blood group systemchemical modifications to a glycoprotein the H antigen on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other IA, IB and dominant over the recessive i at the ABO locus.
The IA and IB alleles produce different modifications. The enzyme coded for by IA adds an N-acetylgalactosamine to the membrane-bound H antigen. The IB enzyme adds a galactose. The i allele produces no modification. The medical condition produced by the heterozygous genotype is called sickle-cell trait and is a milder condition distinguishable from sickle-cell anemiathus the alleles show incomplete dominance with respect to anemia, see above.
For most gene loci at the molecular level, both alleles are expressed co-dominantly, because both are transcribed into RNA. Co-dominance, where allelic products co-exist in the phenotype, is different from incomplete dominance, where the quantitative interaction of allele products produces an intermediate phenotype. For example, in co-dominance, a red homozygous flower and a white homozygous flower will produce offspring that have red and white spots. These ratios are the same as those for incomplete dominance.
Again, note that this classical terminology is inappropriate — in reality such cases should not be said to exhibit dominance at all. Addressing common misconceptions[ edit ] While it is often convenient to talk about a recessive allele or a dominant trait, dominance is not inherent to either an allele or its phenotype.
Dominance is a relationship between two alleles of a gene and their associated phenotypes. A "dominant" allele is dominant to a particular allele of the same gene that can be inferred from the context, but it may be recessive to a third allele, and codominant to a fourth. Similarly, a "recessive" trait is a trait associated with a particular recessive allele implied by the context, but that same trait may occur in a different context where it is due to some other gene and a dominant allele.
Dominance is unrelated to the nature of the phenotype itself, that is, whether it is regarded as "normal" or "abnormal," "standard" or "nonstandard," "healthy" or "diseased," "stronger" or "weaker," or more or less extreme.
A dominant or recessive allele may account for any of these trait types. Dominance does not determine whether an allele is deleterious, neutral or advantageous. However, selection must operate on genes indirectly through phenotypes, and dominance affects the exposure of alleles in phenotypes, and hence the rate of change in allele frequencies under selection.
Deleterious recessive alleles may persist in a population at low frequencies, with most copies carried in heterozygotes, at no cost to those individuals. These rare recessives are the basis for many hereditary genetic disorders. Dominance is also unrelated to the distribution of alleles in the population. Some dominant alleles are extremely common, while others are extremely rare.X-Linked Trait Review
The most common allele in a population may be recessive when combined with some rare variants. Nomenclature[ edit ] This section is about gene notations that identify dominance. For modern formal nomenclature, see Gene nomenclature. In genetics, symbols began as algebraic placeholders. When one allele is dominant to another, the oldest convention is to symbolize the dominant allele with a capital letter.
The recessive allele is assigned the same letter in lower case. In the pea example, once the dominance relationship between the two alleles is known, it is possible to designate the dominant allele that produces a round shape by a capital-letter symbol R, and the recessive allele that produces a wrinkled shape by a lower-case symbol r.
The homozygous dominant, heterozygous, and homozygous recessive genotypes are then written RR, Rr, and rr, respectively. It would also be possible to designate the two alleles as W and w, and the three genotypes WW, Ww, and ww, the first two of which produced round peas and the third wrinkled peas. Note that the choice of "R" or "W" as the symbol for the dominant allele does not pre-judge whether the allele causing the "round" or "wrinkled" phenotype when homozygous is the dominant one.
Recessive Inheritance - Genetics Generation
A gene may have several alleles. Each allele is symbolized by the locus symbol followed by a unique superscript. In many species, the most common allele in the wild population is designated the wild type allele. Other alleles are dominant or recessive to the wild type allele. This equals two matched sets of 22 chromosomes from each parent, called autosomal chromosomes, plus a sex chromosome from each.
The egg from the mother always contributes an X chromosome, while the sperm from the father contributes either a Y or an X chromosome. Each autosomal chromosome carries many genes, and these genes each have a match or allele on the corresponding chromosome from the other parent -- although of course the traits these genes code for can vary. The X chromosome is larger than the Y chromosome and carries more genes, however, so some genes on the X chromosome do not have corresponding alleles on the Y chromosome.
Autosomal Dominant Genes For any pair of alleles, a dominant allele is will exert its influence on the body and override a recessive allele.
For a dominant gene to express itself, only one dominant allele has to be inherited.
A parent with one copy of a dominant allele and one recessive -- coded Ww for the widow's peak -- is called heterozygous for a trait and has a 50 percent chance of passing on the dominant allele to each of his or her offspring, regardless of the gene inherited from the other parent. When both parents are heterozygous for a trait, two out of four children on average will also be heterozygous, one out of four will receive two dominant alleles -- WW -- and be homozygous dominant, and one out of four will receive two recessive alleles -- ww -- and be homozygous recessive.
Since both Ww and WW code for the presence of a widow's peak, however, an average of three out of four offspring will have the trait. Autosomal Recessive Genes Recessive traits are expressed only if the offspring inherits a recessive allele from each parent. If both parents carry only one copy of a recessive gene, the likelihood of a child bearing the recessive trait is one in four.
The probability rises to two in four if one parent is homozygous recessive for the gene -- for example, ww -- and the other parent is heterozygous.