Mendelian Genetics An Overview

Gregor Mendel Austrian Monk. Studied pea plants. Wanted to know why when some purple pea plants were pollinated, their offspring would come out white.

Mendel’s Laws of Inheritance Segregation: In diploid organisms, chromosome pairs (and their alleles) are separated into individual gametes (eggs or sperm) to transmit genetic information to offspring. Dominance: A dominant allele completely masks the effects of a recessive allele. A dominant allele produces the same phenotype in heterozygotes and in homozygotes. Independent assortment: Alleles on different chromosomes are distributed randomly to individual gametes.

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. Fig. 14.1 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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

By the law of segregation, the two alleles for a character are packaged into separate gametes 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 like the purple-flowered parents. Fig. 14.2 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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. Fig. 14.2 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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

Law of Segregation - the two alleles for each character segregate during gamete production

Law of Independent Assortment – Each set of alleles segregates independently

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 an offspring receiving two recessive alleles during fertilization ½ x ½ = ¼

Rule Applies to dihybrid Crosses For a dihybrid cross, YyRr x YyRr, what is the probability of an F2 having the genotype YYRR?

Incomplete Dominance Incomplete dominance refers to a blending of characteristics,

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.

SAMPLE PROBLEM 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.

Epistasis Epistasis occurs when a gene at one locus alters or influences the expression of a gene at a second loci. In this example, C is for color and the dominate allele must be present for pigment (color) to be expressed.

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.

Environmental Impact on Phenotype pH of the soil will change the color of hydrangea flowers from blue to pink

Technology And Genetic testing Fetal testing Amniocentesis Chorionic villus sampling (CVS) Ultrasound Fetoscopy Newborn screening Carrier Recognition