Mendelian Genetics An Overview

• Pea plants have several advantages for genetics. – Pea plants are available in many varieties with distinct

heritable features (characters) with different variants (traits). – Another advantage of peas is that Mendel had strict control

over which plants mated with which. – Each pea plant has male

(stamens) and female (carpal) sexual organs.

– In nature, pea plants typically self-fertilize, fertilizing ova with their own sperm.

– However, Mendel could also move pollen from one plant to another to cross-pollinate plants.

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Fig. 14.1

• In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties. – The true-breeding parents are the P generation and

their hybrid offspring are the F1 generation. • Mendel would then allow the F1 hybrids to self-

pollinate to produce an F2 generation. • It was mainly Mendel’s quantitative analysis of

F2 plants that revealed the two fundamental principles of heredity: the law of segregation and the law of independent assortment.

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• If the blending model were correct, the F1 hybrids from a cross between purple-flowered and white-flowered pea plants would have pale purple flowers.

• Instead, the F1 hybrids all have purple flowers, just a purple as the purple-flowered parents.

2. By the law of segregation, the two alleles for a characters are packaged into separate gametes

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Fig. 14.2

• When Mendel allowed the F1 plants to self-fertilize, the F2 generation included both purple-flowered and white-flowered plants. – The white trait, absent in the F1, reappeared in the

F2. • Based on a large

sample size, Mendel recorded 705 purple-flowered F2 plants and 224 white-flowered F2 plants from the original cross.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 14.2

Vocabulary • Character –heritable feature • Trait – each variant for a character • True-breeding – plants that self-pollinate all

offspring are the same variety • Monohybrid cross – a cross that tracks the

inheritance of a single character • P generation – (parental) true-breeding • F1- (first filial) offspring of P generation • F2 – (second filial) offspring from F1 cross

Vocabulary (continued) • Allele- alternate version of a gene • Dominate allele – expressed in the heterozygote • Recessive allele – not expressed in the heterozygote • Homozygote – pair of identical alleles for a

character – Homozygous dominant- BB – Homozygous recessive - bb

• Heterozygote – two different alleles for a character (Bb)

• Genotype – genetic makeup • Phenotype – appearance of an organism

Test cross – designed to reveal the genotype of an organism

Mendelian Inheritance and Rules of Probability

• Rule of Multiplication – the probability that two events will occur simultaneously is the product of their individual probabilities

• Probability that an egg from the F1 (Pp) will receive p = ½

• Probability that an sperm from the F1 (Pp) will receive p = ½

• Probability that a of offspring receiving two recessive alleles during fertilization ½ x ½ = ¼

Incomplete Dominance

Codominance

• Two alleles affect the phenotype in separate and distinguishable ways.

• Neither allele can mask the other and both are expressed in the offspring and not in an “intermediate” form.

• Example: red flowers that are crossed with white flowers that yield red and white flowers.

• 1) In cattle, roan coat color (mixed red and white hairs) occurs in the heterozygous (Rr) offspring of red (RR) and white (rr) homozygotes. When two roan cattle are crossed, the phenotypes of the progeny are found to be in the ratio of 1 red:2 roan:1 white. Which of the following crosses could produce the highest percentage of roan cattle?

• A) roan x roan • B) red x white • C) white x roan • D) red x roan • E) All of the above crosses would give the same

percentage of roan.

Multiple Alleles

Pleiotropy

• Most genes have multiple phenotypic effects. The ability of a gene to affect an organism in many ways is called pleiotropy.

Polygenetic Inheritance • Qualitative variation

usually indicates polygenic inheritance. This occurs when there is an additive effect from two or more genes. Pigmentation in humans is controlled by at least three (3) separately inherited genes.