Unit 3 – Chapters 10 and 12 Mendel, Meiosis, and Genetics

Chapter 10 and 12 Objectives 1.Describe the steps involved in Mendel’s experiments on garden peas. 2.Predict the possible offspring of a genetic cross by using a Punnett square 3.Analyze how meiosis maintains a constant number of chromosomes within a species 4.Infer how meiosis leads to variation in a species

Predict how a human trait can be determined by a simple dominant allele Distinguish between incompletely dominant and codominant alleles Analyze the pattern of sex-linked inheritance

Gregor Mendel carried out the first important studies of heredity; how one’s parents influence how the offspring’s characteristics occur. Thus, the branch of biology called genetics was created. The characteristics that are inherited from one generation to the next are called traits.

In plants, the male gamete is the pollen, the female gamete is the ovule, located in the pistil. The transfer of the pollen to the pistil is called pollination. The uniting of the male and female gametes is called fertilization.

Mendel studied Seven Traits or Characteristics. Each characteristic occurred in Two Contrasting Traits:

Mendel crossed tall pea plants with short pea plants. These were called the P1 generation (parent – 1) When the offspring grew, they were all tall. This was called the F1 generation (Filial – son or daughter -1)

When these plants were crossed, three out of four plants were tall, one was short. This was called the F2 generation. Mendel used seven traits in all to study – one at a time. In each case, he found that one trait disappeared in the F1 generation, only to reappear in the F2 generation.

Mendel concluded that each organism has two genes that control each trait. We later determined that these are on the chromosomes and exist in alternative forms called alleles.

Each allele is located on different copies of a chromosome, one inherited from the female parent, and one inherited from the male parent. Since only one of the two traits was observed, that trait is known as the dominant trait. The one that is not observed is called the recessive trait.

Letters are used to represent Alleles: Capital Letters refer to a Dominant Allele that masks or hides expression of a recessive allele. Lowercase Letters refer to a Recessive Allele its expression is masked by a dominant allele.

From these results, Mendel formulated the first of two laws of heredity: The Law of Segregation. This consists of the theory of inheritance because many individual factors are passed on from generation to generation.

Because Mendel had demonstrated that there were two different types of tall plants, the P1 and the F1, he needed a way to distinguish between the two. The way a plant looks; tall, is called the phenotype. However, since it can be either TT or Tt, the gene type of an organism is called its genotype.

You can not tell by looking at the plant whether it is TT which is called homozygous dominant; or is it Tt which is called heterozygous. Homozygous is when the trait is the same, whether it is the dominant or the recessive trait. Heterozygous tells us that the trait is represented with both the dominant and the recessive gene.

If the organism is showing the recessive gene, tt, the phenotype of short also gives you the genotype because recessive traits do not show if the dominant trait is present.