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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 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 F 1 generation. Mendel would then allow the F 1 hybrids to self- pollinate to produce an F 2 generation. It was mainly Mendel’s quantitative analysis of F 2 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

If the blending model were correct, the F 1 hybrids from a cross between purple-flowered and white- flowered pea plants would have pale purple flowers. Instead, the F 1 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 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 14.2

When Mendel allowed the F 1 plants to self- fertilize, the F 2 generation included both purple- flowered and white-flowered plants. –The white trait, absent in the F 1, reappeared in the F 2. Based on a large sample size, Mendel recorded 705 purple-flowered F 2 plants and 224 white-flowered F 2 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 F 1 - (first filial) offspring of P generation F 2 – (second filial) offspring from F 1 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 a of offspring receiving two recessive alleles during fertilization ½ x ½ = ¼

Dihybrid crosses

Two factor crosses

Rule Applies to dihybrid Crosses For a dihybrid cross, YyRr x YyRr, what is the probability of an F2 having the genotype YYRR? Dihybrid Crosses: Crosses that involve 2 traits..

These type of crosses can be challenging to set up, and the square you create will be 4x4. This simple guide will walk you through the steps of solving a typical dihybrid cross common in genetics. The method can also work for any cross that involves two traits

Consider this cross A pea plant that is heterozygous for round, yellow seeds is self fertilized, what are the phenotypic ratios of the resulting offspring? Step 1: Determine the parental genotypes from the text above, the word "heteroyzous" is the most important clue, and you would also need to understand that self fertilized means you just cross it with itself. RrYy x RrYy

Step 2: Determine the gametes. This might feel a little like the FOIL method you learned in math class. Combine the R's and Ys of each parent to represent sperm and egg. Do this for both parents Gametes after "FOIL" RY, Ry, rY, ry (parent 1) and RY, Ry, rY, ry (parent 2) Step 3: Set up a large 4x4 Punnet square, place one gamete set from the parent on the top, and the other on the side

Step 4: Write the genotypes of the offspring in each box and determine how many of each phenotype you have. In this case, you will have 9 round, yellow; 3 round, green; 3 wrinkled, yellow; and 1 wrinkled green Some Shortcuts In any case where the parents are heterozygous for both traits (AaBb x AaBb) you will always get a 9:3:3:1 ratio. 9 is the number for the two dominant traits, 3 is the number for a dominant/recessive combination, and only 1 individual will display both recessive traits. Another way to determine the ratios is to do it mathematically 3/4 of all the offspring will have round seeds 3/4 of all the offspring will have yellow seeds 3/4 x 3/4 = 9/16 will have round, yellow seeds. Crosses that Involve 2 Traits Consider: RrYy x rryy The square is set up as shown You might notice that all four rows have the same genotype. In this case, you really only need to fill out the top row, because 1/4 is the same thing as 4/16

Incomplete Dominance Page 267 and work #2

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 Page 267 and work #6

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 Carrier Recognition Newborn screening Fetal testing 1.Amniocentesis 2.Chorionic villus sampling (CVS) 3.Ultrasound 4.Fetoscopy