1. Mendelian Genetics
An Overview

3. 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
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4. 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

5. 2. By the law of segregation, the two alleles for characters 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 a purple as the purple-flowered parents.
Fig. 14.2
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

6. 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

8. 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

9. Vocabulary (continued)
Allele- alternate version of a gene
Dominant 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

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

12. Law of Independent Assortment – Each set of alleles segregates independently

13. Test cross – designed to reveal the genotype of an organism

14. 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

15. Rule Applies to dihybrid Crosses
For a dihybrid cross, YyRr x YyRr, what is the probability of an F2 having the genotype YYRR?
Go page 267 and work #9 and #10

16. Incomplete Dominance
Page 267 and work #2

17. 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.

18. 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.

19. Multiple Alleles
Page 267 and work #6

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

21. 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.

22. 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.

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

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