Assignment 9: Genetics II: Solving Genetics Problems
ACTIVITY 1. SEX DETERMINATION AND SEX-LINKED GENES
In biological terms, sex is about mixing DNA material from two individuals in an organized fashion. In this exercise, will focus on multicellular, diploid organisms in which sex entails a distribution of chromosomes to reproductive cells via meiosis.
In constructing Punnett squares to tabulate chromosome distribution to gametes (assignment 8), we have assumed that all paired chromosomes are homologous. This is true for all chromosome pairs in some animals and plants. But In many organisms (including most animals) there is a chromosome pair known as sex chromosomes which are not homologous; the pattern of inheritance of these chromosomes dictates the sex of the organism. In these cases, the pair of sex chromosomes are not completely homologous and often differ in size. The remaining chromosome pairs are homologous and are called autosomes.
The pattern of sex determination that you are most familiar with is that of mammals in which the pair of sex chromosomes consists of a large chromosome (called X) and a much smaller chromosome (called Y). Although these chromosomes are not completely homologous, they do pair during meiosis. An individual that inherits two Xs develops into a female, whereas an individual inheriting one X and one Y develops into a male. There are other animal groups, such as some insects, which also have an XY system. However, some of our closer relatives (groups of vertebrate animals) have different systems. For example, the female may have the odd pair of chromosomes, while the male has two homologous sex chromosomes. This system is known as ZW. In other cases, sex is not determined by genotype at all, but by environmental factors! Read the following descriptions of alternate sex determination systems, then answer question 1.
While it is important to remember that not all organisms have the XY system for sex determination, we will now focus on the mammalian system. It is important that you become familiar with this sex-determination system because the larger X chromosome contains genes for traits which are not found on the Y chromosome, and some human disorders (such as hemophilia and colorblindness) are determined by these alleles. Traits in which the allele is found on the X, but not the Y, chromosome are known as sex-linked traits. When working with sex linkage, it is important to follow the inheritance of the sex chromosomes and keep in mind that males have only one X and thus only one allele for sex-linked traits.
In the Punnett squares below, B is the allele for normal red-green color vision and b is the defective allele. These alleles are only present on the X chromosome and the defective allele is recessive. Compare the two crosses and note that when one parent is colorblind and the other has normal color vision, the sex of the colorblind parent determines whether any offspring will be colorblind.
(Would this be true of the mother was heterozygous for B?)
ACTIVITY 2. INDEPENDENT CHROMOSOME ASSORTMENT VS. LINKED GENES
Independent assortment and dihybrid crosses
Independent assortment refers to the way that chromosomes pairs move during meiosis. Each homologous pair comes to the metaphase plate as an independent entity. Hence the pairs assume random alignments (positions) relative to each other. Review diagram 2 on last week's help sheet to be sure that you understand the ways in which two pairs of homologous chromosomes can be distributed to gametes.
Now fill in diagram 1 on this week's help sheet. It is the same as the previous diagram, except that the chromosomes are labeled A/a and B/b. Determine for yourself that one alignment of two chromosome pairs gives gametes AB and ab, whereas the alternate alignment gives gives gametes Ab and aB. Please do this, even though you are not required to submit the diagram; working with these help sheets is the key to understanding how meiosis relates to dihybrid crosses. You may wish to review this animation on independent chromosome assortment (which uses 3 homologous pairs) before filling out the diagram.
Now you are ready to examine the inheritance of two alleles that reside on different chromosomes. We will begin with the most complex scenario in which the parents are heterozygous for both traits. Observe how the following Punnett square is set up to show all possible gametes. (Note that Punnett square analysis only works for gene pairs that are on non-homologous chromosomes.)
Use the Punnett square on your help sheet to fill in the genotypes of the possible offspring from this dihybrid cross. When you understand how independent assortment generates gametes and how to set up Punnett squares for two traits, answer questions 6 and 7.