Answers to Hardy-Weinberg practice questions Updated: The frequency of the "aa" genotype.
Introduction to genetics The concept of dominance was introduced by Gregor Johann Mendel.
Where ‘p 2 ‘ represents the frequency of the homozygous dominant genotype (AA), ‘2pq‘ the frequency of the heterozygous genotype (Aa) and ‘q 2 ‘ the frequency of the homozygous recessive genotype (aa). The sum of these three genotypes must equal 1 (%). The recessive phenotype is controlled by the homozygous aa genotype. Therefore, the frequency of the dominant phenotype equals the sum of the frequencies of AA and Aa, and the recessive phenotype is simply the frequency of aa. q 2 = frequency of aa (homozygous recessive) The following is an example of using the Hardy-Weinberg equation to predict carrier frequency: Phenylketonuria (PKU) is an autosomal recessive metabolic disorder that results in mental retardation if untreated during the newborn period.
Though Mendel, "The Father of Genetics", first used the term in the s, it was not widely known until the early twentieth century. Mendel observed that, for a variety of traits of garden peas having to do with the appearance of seeds, seed pods, and plants, there were two discrete phenotypes, such as round versus wrinkled seeds, yellow versus green seeds, red versus white flowers or tall versus short plants.
When bred separately, the plants always produced the same phenotypes, generation after generation.
However, when lines with different phenotypes were crossed interbredone and only one of the parental phenotypes showed up in the offspring green, or round, or red, or tall.
However, when these hybrid plants were crossed, the offspring plants showed the two original phenotypes, in a characteristic 3: Mendel reasoned that each parent in the first cross was a homozygote for different alleles one parent AA and the other parent aathat each contributed one allele to the offspring, with the result that all of these hybrids were heterozygotes Aaand that one of the two alleles in the hybrid cross dominated expression of the other: Mendel did not use the terms gene, allele, phenotype, genotype, homozygote, and heterozygote, all of which were introduced later.
He did introduce the notation of capital and lowercase letters for dominant and recessive alleles, respectively, still in use today. Chromosomes, genes, and alleles[ edit ] See also: Ploidy and Zygosity an autosomal dominant pattern.
Most animals and some plants have paired chromosomesand are described as diploid. They have two versions of each chromosome, one contributed by the mother's ovumand the other by the father's spermknown as gametesdescribed as haploid, and created through meiosis.
These gametes then fuse during fertilization during sexual reproductioninto a new single cell zygotewhich divides multiple times, resulting in a new organism with the same number of pairs of chromosomes in each non-gamete cell as its parents. Each chromosome of a matching homologous pair is structurally similar to the other, and has a very similar DNA sequence locisingular locus.
The DNA in each chromosome functions as a series of discrete genes that influence various traits. Thus, each gene also has a corresponding homologue, which may exist in different versions called alleles.
The alleles at the same locus on the two homologous chromosomes may be identical or different. The blood type of a human is determined by a gene that creates an A, B, AB or O blood type and is located in the long arm of chromosome nine.
There are three different alleles that could be present at this locus, but only two can be present in any individual, one inherited from their mother and one from their father.
The genetic makeup of an organism, either at a single locus or over all its genes collectively, is called its genotype. The genotype of an organism directly and indirectly affects its molecular, physical, and other traits, which individually or collectively are called its phenotype.
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.Now, if A is completely dominant to a then the frequency of the carrier genotype Aa cannot be directly observed (since it has the same traits as the homozygous genotype AA), however it can be estimated from the frequency of the recessive trait in the population, since this is the same as that of the homozygous genotype aa.
The recessive phenotype is controlled by the homozygous aa genotype. Therefore, the frequency of the dominant phenotype equals the sum of the frequencies of AA and Aa, and the recessive phenotype is simply the frequency of aa.
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, p = 1 − q, and from those the frequency of the carrier genotype can be derived: f(Aa) = 2pq.
If the frequency of a homozygous dominant genotype is , what is the frequency of the homozygous recessive genotype? Biology Genetics & Inheritance Hardy-Weinberg Equation.
1 Answer Let's re-write Hardy Weinberg using . Genotypes and phenotypes brings us to the terms genotype, phenotype, and trait. An organism can be homozygous dominant, if it carries two copies of the same dominant allele, or homozygous recessive, if it carries two copies of the same recessive allele.
2pq is the frequency of the heterozygous genotype (Aa) In the Hardy-Weinberg equation p²+2pq+q² = 1, what does q² stand for? q² stands for the frequency of the homozygous recessive genotype (aa).