Genetics Similarity And Variation

Genetics Similarity and variation

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The Laws of Heredity Mendel’s 1st Law - The Law of Segregation The first law, the law of segregation, states that the two alleles for a trait segregate (separate) when gametes are formed. The first law of heredity describes the behavior of chromosomes during meiosis At this time, homologous chromosomes and then chromatids are separated. resulting in a 3:1 ratio between dominant and recessive phenotypes Observations: 1. The F1 derived from the crosses between two different varieties showed only one of the traits and never the other. 2. The trait that was hidden in F1, reappeared in F2. 3. Reciprocal crosses gave the same result. 4

Mendel’s 2nd Law – Law of Independent Assortment The law of independent assortment states that the alleles of different genes separate independently of one another during gamete formation. The inheritance of one trait did not influence the inheritance of any other trait. his experiments with mixing two traits (dihybrid cross) showed 9:3:3:1 ratios. But the 9:3:3:1 ratio shows that each of the two genes are independently inherited with a 3:1 ratio. Mendel concluded that different traits are inherited independently of each other, so that there is no relation, for example, between a flower colour and seed shape. This is actually only true for genes that are not linked to each other. 5

Variations on Mendel’s Laws 1. Incomplete Dominance 2. Codominance 3. Multiple Alleles 4. Pleiotropy 5. Epistasis 6. Polygenic Inheritance 7. Environmental Effects

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Incomplete Dominance

In some organisms, however, an individual displays a trait that is intermediate between the two parents, a condition known as incomplete dominance (intermediate expression). If there are only two alleles involved in determining the phenotype of a certain trait, but there are three possible phenotypes, then the inheritance of the trait illustrates either incomplete dominance or codominance. In these situations a heterozygous (hybrid) genotype produces a 3rd phenotype that is either a blend of the other two phenotypes (incomplete dominance) or a mixing of the other phenotypes with both appearing at the same time (codominance)(both are expressed).

Ex- when a snapdragon with red flowers is crossed with a snapdragon with white flowers, a snapdragon with pink flowers is produced 7

P Generation Red CRCR

White CW CW

Gametes CR

CW

Pink CRCW

F1 Generation

Gametes 1

/2

1

F2 Generation

CR

W /2 C

1

Sperm /2 CR

/ 2 CW

1

/ 2 CR

1

CR

Eggs /2 CW CRCR

1

CR CW 8

For some traits, two dominant alleles are expressed at the same time. In this case, both forms of the trait are displayed, a phenomenon called codominance. Codominance is different from incomplete dominance because both traits are displayed. Characters with two forms displayed at the same time: For some traits, two dominant alleles are expressed at the same time. In this case, both forms of the trait are displayed, a phenomenon called codominance. Codominance is different from incomplete dominance because both traits are displayed. 9

Genes with three or more alleles are said to have multiple alleles. Now, if there are 4 or more possible phenotypes for a particular trait, then more than 2 alleles for that trait must exist in the population.  We call this "MULTIPLE ALLELES".

Even for traits controlled by genes with multiple alleles, an individual can have only two of the possible alleles for that gene. Multiple alleles originate because of changes in nucletides in the DNA sequence.

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Allele IA

Carbohydrate A

B IB Multipl i none e (a) The three alleles for the ABO blood groups allele and their associated carbohydrates s Genotype

Red blood cell appearance

IAIA or IA i IBIB or IB i

Phenotype (blood group) A

B

IA IB

AB

ii

O

(b) Blood group genotypes and phenotypes

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