3.1 | Biology homework help

 

This module requires you to read carefully.

Go to the suggested websites to help in understanding how to set up and perform the math for these kinds of genetic problems

http://www.biology.arizona.edu/mendelian_genetics/problem_sets/monohybrid_cross/monohybrid_cross.html

http://www.biology.arizona.edu/mendelian_genetics/problem_sets/dihybrid_cross/dihybrid_cross.html

The lab link:   http://www.mhhe.com/biosci/genbio/virtual_labs/BL_05/BL_05.html

 The lab is to help you learn how to use the Punnett Square approach;

Here is a blurb about it from the site: How can Punnett squares help predict the traits of offspring?

Heredity is the passing of traits from parent to offspring. Characters are controlled by genes. The different forms a gene may have for a trait are called alleles. There are two alleles for every trait. For example, Mendel’s pea plants had two alleles for flower color—an allele for the purple trait and an allele for the white one. Mendel crossed a purple-flowered pea plant with a white-flowered pea plant. All the offspring of this cross had purple flowers. Mendel determined that the purple flower allele was dominant because it covered up the white flower allele, which he called recessive.

             For example, Mendel’s pea plants had two alleles for flower color—an allele for the purple trait and an allele for the white one. Mendel crossed a purple-flowered pea plant with a white-flowered pea plant.

All the offspring of this cross had purple flowers. Mendel determined that the purple flower allele was dominant because it covered up the white flower allele, which he called recessive.

English biologist Reginald Punnett developed a simple method for predicting how alleles can combine. It is called a Punnett square. In a Punnett square, dominant and recessive alleles are represented by letters. An uppercase letter represents a dominant allele, and a lowercase letter represents a recessive allele. Each cell in an organism’s body contains two alleles for every trait. One allele is inherited from the female parent, and one allele is inherited from the male parent.

An organism is homozygous if it has identical alleles for a particular trait. An organism is heterozygous if it has nonidentical alleles for a particular trait. There are three possible combinations of alleles of an organism for a particular trait: homozygous dominant (PP), heterozygous (Pp), and homozygous recessive (pp). 

If only one trait is considered in a genetic cross, the cross is called monohybrid. A Punnett square for a monohybrid cross is two boxes tall and two boxes wide because each parent can pass on two kinds of alleles. For example, a heterozygous parent (Pp) can pass on the (P) allele or the (p) allele. The first box of the Punnett square is labeled (P) and the second box in the Punnett square is labeled (p).

If two traits are being considered in a genetic cross, the cross is called dihybrid. A Punnett square for a dihybrid cross is four boxes tall and four boxes wide because each parent can pass on four possible combinations of alleles. For example, a heterozygous parent (PpTt) can pass on the following combinations of alleles: (PT), (Pt), (pT), (pt). Each box of the Punnett square is labeled with one of these possible combinations of alleles.

Alleles represent a genotype or the genetic makeup of an organism for a trait. A phenotype is a physical trait that shows as a result of an organism’s particular genotype. Organisms may look alike but have different genotypes. It is impossible to determine homozygous dominant and heterozygous genotypes by sight.

Common fruit flies, Drosophila melanogaster, are often used to study genetics because of their short life cycle (about 2 weeks), their small size (several hundred can be housed in a small jar), and their high rate of reproduction (a single pair may produce more than a hundred offspring).

When using Punnett squares to analyze the results of genetic crosses involving body color in Drosophila, “G” represents the dominant allele for gray body color and “g” represents the recessive allele for black body color. When using Punnett squares to analyze the results of genetic crosses involving wing length in Drosophila, “L” represents long wing length and “l” represents vestigial wing length. (Vestigial wings are wings with a reduced function that may have been useful to an earlier evolutionary stage of Drosophila.)