Mendelian Genetics Study Guide
Chapter 11
11-1 to 11-3
A trait is a specific characteristic such as seed color or plant height. Each trait is controlled by at least one gene. Each adult usually has two copies of each gene, one from each parent. Each gene is often found in two contrasting forms, such as tall and short. Each form is called an allele.
Gregor Mendel is called the father of genetics because he developed two key genetic principles by studying pea plants which apply in general to eukaryotes.
The principle of dominance states that some alleles are usually dominant and others are recessive. Occasionally some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.
Each gamete (sex cell) carries only one allele of the gene. This is called segregation. Segregation occurs during meiosis.
The principle of independent assortment states that genes for different traits usually segregate independently during the formation of gametes. Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms. The exception to independent assortment is when genes are linked on the same chromosome. (Segregation and independent assortment are easy to confuse).
Punnet Squares can be used to predict the probability of the phenotypes (physical characteristics) and genotypes (genetic makeup) of parent P, first filial F1, and second filial F2 generations.
The type of gametes are shown along the top and left side of the square. The possible gene combinations for the offspring appear in the boxes that make up the center of the square. The letters in the Punnet square represents alleles. Capitals represent dominant alleles, small letters recessive alleles.
Homozygous: organisms that have two identical alleles for a particular trait. Organisms that have two different alleles for the same trait are heterozygous.
Figure 11-7 Punnett square is an example of a monohybrid cross between heterozygous tall (Tt) pea plants. The phenotype outcome is 3:1 tall to short plants. Figure 11-9 and 11-10 are examples of dihybrid crosses for the F1 and the F2 generation. This shows the principle of independent assortment. The F2 phenotype is 9:3:3:1.
An example of incomplete dominance is the four o’clock plant. The heterozygous (RW) plants of the F1 generation have pink flowers when a homozygous WW white parent is crossed with a homozygous RR red parent.
11-4
A set of chromosomes in an adult cell is usually made of homologous pairs. Homologous pairs are corresponding chromosomes, one from the male parent and the other from the female parent.
A cell containing both homologous sets is called a diploid cell (2N).
Meiosis is a process of cell division in which the number of chromosomes is cut in half through the separation of homologous chromosomes in a diploid cell. A haploid set of chromosomes is formed containing only one single set of chromosomes (N), and therefore a single set of genes. Within the set, some chromosomes are from the male parent, others are from the female parent, so this increases the diversity of the offspring. For each cell undergoing meiosis, four haploid cells are produced. Gametes are formed from one or more of these cells.
Crossing over during meiosis increases the genetic diversity even more. This results in the exchange of a few alleles between homologous chromosomes, and produces new combinations of alleles.
Mitosis results in two genetically identical diploid cells, where meiosis produces four genetically different haploid cells.