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Chapter 5: Problem 20

If there are five alleles at a locus, how many genotypes can there be at this locus? How many different kinds of homozygotes can there be? How many genotypes and homozygotes can there be with eight alleles at a locus?

Short Answer

Expert verified
With five alleles, 15 genotypes and 5 homozygotes; with eight alleles, 36 genotypes and 8 homozygotes.

Step by step solution

01

Understanding Alleles and Genotypes

An allele is a variant of a gene at a given locus. Genotypes refer to the genetic makeup consisting of two alleles at a locus.
02

Determining Genotype Combinations for Five Alleles

When there are five alleles (A, B, C, D, E), genotypes are formed by pairing each allele with any of the same or different alleles. The formula to calculate the number of genotypes is \( \frac{n(n+1)}{2} \), where \( n \) is the number of alleles. For five alleles, the total number of potential genotypes is \( \frac{5 \times 6}{2} = 15\).
03

Determining Homozygous Combinations for Five Alleles

A homozygote consists of two identical alleles (e.g., AA, BB). For five alleles, each allele can form one homozygous combination, resulting in 5 different homozygotes.
04

Determining Genotype Combinations for Eight Alleles

Using the formula for genotypes where \( n = 8 \), the number of genotypes is \( \frac{8 \times 9}{2} = 36\).
05

Determining Homozygous Combinations for Eight Alleles

Similarly, for eight alleles, there are 8 homozygous genotypes, one for each allele (e.g., AA, BB, ..., HH).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Alleles
Imagine you're flipping through a catalog of your genes. Alleles are like the different styles or versions of one particular item in this catalog. They represent the various forms a gene might take. For instance, if you have a gene that determines eye color, one allele might lead to blue eyes while another might lead to brown eyes.
  • Each allele occupies the same spot on a chromosome, known as a gene locus.
  • Since organisms usually receive one allele from each parent, they typically have a pair of alleles at each gene locus.
  • Different combinations of these alleles can lead to different genetic traits.
Understanding alleles is key in genetics because these are the building blocks that dictate how traits are passed from one generation to the next.
Genotypes
Genotypes are essentially the unique combination of alleles that determine specific traits in an organism. Think of them as your genetic recipe card that details exactly what ingredients (alleles) are used at each gene locus.
  • The genotype gives rise to observable characteristics, known as the phenotype.
  • In many cases, organisms inherit two alleles for each gene locus - one from each parent.
  • This combination can be either homozygous, where the two alleles are identical, or heterozygous, where they differ.
Exploring genotypes allows scientists to predict the likelihood of certain traits manifesting, providing insights into the hereditary patterns of organisms.
Homozygotes
A homozygote is an organism where both alleles at a specific gene locus are identical. Picture a pair of identical twins, standing side by side - that鈥檚 a simple way to visualize a homozygous pair of alleles.
  • Homozygous organisms express traits based on these alleles consistently.
  • There are two forms: homozygous dominant (e.g., AA) and homozygous recessive (e.g., aa).
  • For example, a plant with homozygous alleles for flower color might always produce red flowers, assuming red is the dominant trait.
Homozygosity plays a crucial role in genetics, helping predict whether certain traits will be consistently passed down to future generations.
Gene Locus
A gene locus is the precise location on a chromosome where a gene resides. Visualize a library where each book (gene) has a designated spot on a shelf (chromosome); the specific spot is the locus of that book.
  • The locus of a gene remains constant across individuals within a species, ensuring consistency in genetic mapping.
  • Every allele of a gene will exist at the same locus on the homologous chromosome.
  • It serves as an anchor point for understanding how genes are organized and interrelated.
Comprehending the concept of a gene locus is vital for geneticists as it simplifies the understanding of complex genetic interactions along the strands of DNA.

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Most popular questions from this chapter

White eyes is an X-linked recessive mutation in Drosophila that results in eyes that are devoid of the normal red pigment seen in wild-type flies (see Chapter 4 ). The white locus encodes an ABC transporter protein that, when functional, moves a variety of substances across the cell membrane including pigment precursors. When the transporter protein is defective due to a mutation at the white locus, the precursors are not present inside the cell and no eye pigments are produced. Mutations at the white locus also affect mating behavior and the fly's ability to recover from oxygen deprivation. a. What phenomenon is illustrated by the different phenotypic effects of mutations at the white locus? b. Propose an explanation for why mutations at the white eye locus have such differing effects as eye color, mating behavior, and physiology.

A summer-squash plant that produces disc-shaped fruit is crossed with a summer-squash plant that produces long fruit. All the \(\mathrm{F}_{1}\) have disc-shaped fruit. When the \(\mathrm{F}_{1}\) are intercrossed, \(\mathrm{F}_{2}\) progeny are produced in the following ratio: \(9 / 16\) disc- shaped fruit : \(6 / 16\) spherical fruit \(: 1 / 16\) long fruit. Give the genotypes of the \(\mathrm{F}_{2}\) progeny.

The \(L^{\mathrm{M}}\) and \(L^{\mathrm{N}}\) alleles at the MN blood-group locus exhibit codominance. Give the expected genotypes and phenotypes and their ratios in progeny resulting from the following crosses. a. \(L^{\mathrm{M}} L^{\mathrm{M}} \times L^{\mathrm{M}} L^{\mathrm{N}}\) b. \(L^{\mathrm{N}} L^{\mathrm{N}} \times L^{\mathrm{N}} L^{\mathrm{N}}\) c. \(L^{\mathrm{M}} L^{\mathrm{N}} \times L^{\mathrm{M}} L^{\mathrm{N}}\) d. \(L^{\mathrm{M}} L^{\mathrm{N}} \times L^{\mathrm{N}} L^{\mathrm{N}}\) e. \(L^{\mathrm{M}} L^{\mathrm{M}} \times L^{\mathrm{N}} L^{\mathrm{N}}\)

What is gene interaction? What is the difference between an epistatic gene and a hypostatic gene?

In 1983 , a sheep farmer in Oklahoma noticed in his flock a ram that possessed increased muscle mass in his hindquarters. Many of the offspring of this ram possessed the same trait, which became known as the callipyge phenotype (callipyge is Greek for "beautiful buttocks"). The mutation that caused the callipyge phenotype was eventually mapped to a position on the sheep chromosome 18 . When the male callipyge offspring of the original mutant ram were crossed with normal females, they produced the following progeny: \(1 / 4\) male callipyge, \(1 / 4\) female callipyge, \(1 / 4\) male normal, and \(1 / 4\) female normal. When the female callipyge offspring of the original mutant ram were crossed with normal males, all the offspring were normal. Analysis of the chromosomes of these offspring of callipyge females showed that half of them received a chromosome 18 with the allele encoding callipyge from their mother. Propose an explanation for the inheritance of the allele for callipyge. How might you test your explanation?
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