Sunday, November 23, 2008

More on Mendel's Laws

Mendel’s First Law
The Law of Segregation
This law states that the alleles in a pair segregate into different gametes during gamete formation. Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of the organism making the gamete. In terms of chromosomes, this segregation corresponds to the distribution of the two members of a homologous pair of chromosomes to different gametes during Meiosis.

Mendel’s Second Law
The Law of Independent Assortment
This law states that each pair of alleles segregates independently of each other pair of alleles during gamete formation. This law applies only to allele pairs located on different chromosomes (chromosomes that are not homologous).

Mendel performed dihybrid crosses (mating of parent plants that differ in two traits) in plants that were true-breeding for two traits. For example, a plant that had green pod color and yellow seed color was cross-pollinated with a plant that had yellow pod color and green seeds. In this cross, the traits for green pod color (GG) and yellow seed color (YY) are dominant. Yellow pod color (gg) and green seed color (yy) are recessive. The resulting offspring or F1 generation were all heterozygous for green pod color and yellow seeds (GgYy).

Mendel then allowed all of the F1 plants to self-pollinate. He referred to these offspring as the F2 generation. Mendel noticed a 9:3:3:1 ratio. About 9 of the F2 plants had green pods and yellow seeds, 3 had green pods and green seeds, 3 had yellow pods and yellow seeds and 1 had a yellow pod and green seeds.

Mendel performed similar experiments focusing on several other traits like seed color and seed shape, pod color and pod shape, and flower position and stem length. He noticed the same ratios in each case. From these experiments Mendel formulated what is now known as Mendel's law of independent assortment. This law states that allele pairs separate independently during the formation of gametes. Therefore, traits are transmitted to offspring independently of one another.

Chapter 15
The Chromosomal Basis of Inheritance
In the early 1900’s, the chromosome theory of heredity was formed. It stated that genes have specific locations (called loci) on chromosomes, and that it is chromosomes that segregate and assort independently.

After the chromosome theory of heredity was formed, Thomas Hunt Morgan discovered a sex-linked gene. A sex-linked gene is one located on a sex chromosome (x or y in humans). Non sex-linked genes found on non-sex chromosomes are called autosomes.

Review Questions

1.) The fact that all seven of the garden pea traits studied by Mendel obeyed the law of Independent Assortment means that the
a.) Haploid number of garden peas is 7.
b.) Diploid number of garden peas is 7.
c.) Seven pairs of alleles determining these traits are on the same pair of homologous chromosomes.
d.) Formation of gametes in plants is by mitosis only.
e.) Seven pairs of alleles determining these traits behave as if they are on different chromosomes.
2.) In dihybrid crosses, the phenotypic ratio is always…
a.) 1:2:2:1
b.) 3:1
c.) 9:3:3:1
d.) 3:9:9:1
3.) The Law of Segregation corresponds to which of the following?
a.) Meiosis
b.) Mitosis
c.) Cytokinesis
d.) DNA Replication

Answers: e,c,a

Sorry for the delay. Hope you all had a good weekend. Remember we have chapter 15 vocabulary quiz tomorrow and your edited essay is due.

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