1. Mendelian Genetics 5B

2. Genetic Terminology
gene ~ Mendel’s factors; units of heredity; sections of DNA capable of causing a trait dominant ~ the trait which , when in the presence of contrasting trait, expresses itself (dominant gene, dominant characteristic, dominant genotype, dominant phenotype) recessive ~ the trait which, when in the presence of the dominant, does not express itself (recessive gene, recessive characteristic, etc,) segregation ~ the separation of gene pairs (chromosomes) during the formation of gametes (meiosis) so that only single genes are found in the gametes. phenotype ~ the physical expression of an organism’s genes (what an organism looks like ~ short, tall, green, yellow, etc.) genotype ~ the genetic make-up of an individual organism (a description of its genes ~ Tt, tt, homozygous pure, heterozygous, etc.) locus ~ the particular site on a chromosome where a particular gene is located.

3. Genetic Terminology
allele ~ a pair of genes that have the same position on homologous chromosomes homozygous ~ when both alleles in an organism are the same (e.g., tt, TT) heterozygous ~ when both alleles in an organism are not the same (e.g., Tt) monohybrid cross ~ a genetic cross dealing with only one set of characteristics (e.g., tall and short , or green and yellow, or round and wrinkled) Punnett Square ~ a diagram used to show the possible gamete combinations from a genetic cross pedigree ~ a diagram used to show how a specific characteristic is passed through several generations in a family test cross ~ the mating of an organism possessing a dominant phenotype but unknown genotype with an organism possessing a recessive phenotype (and thus a known genotype)

4. Gregor Mendel He was an Austrian monk but also studied science
Spent a lot of time studying heredity in pea plants
Paper published on heredity in 1865
Became known as the Father of Genetics
His work is important to the understanding of modern genetic theories

5. Plant Anatomy
Stamen consists of a stalk (filament) tipped by an anther
Anther is where pollen is produced via meiosis
Pollen grains house cells that develop into sperm
Carpel or pistil consists of a long slender neck (style) with a sticky stigma at its tip
Stigma catches the pollen
At the base of the carpel is the ovary which contains one or more ovules.
Each ovule contains a developing egg

6. Plant Reproduction
A lot of plants, including pea plants, have both male and female reproductive parts and can self pollinate
Cross pollination is when the male gamete comes from a separate plant and fertilizes the egg
Mendel was able to selectively breed the pea plants by removing the stamen (avoiding self pollination) and supplying pollen from a plant of his choice

7. Pea plants have many opposing characteristics
Why Pea Plants?
Pea plants have many opposing characteristics

8. Plant Generations
Parent plants (P1) ~ the first plants crossed in a studied cross. Mendel breed plants until they were true breeding. This means they only produced plants that exhibited the same characteristic as them.
First filial generation (F1) ~ the offspring from the first cross of the parent plants. These individuals are hybrids.
Second filial generation (F2) the offspring resulting from the crossing of the F1 plants

9. Analyze the data.
What do you observe?

11. Mendel’s biggest question was how does one characteristic go away in the F1 generation but then return in the F2 generation?
There is an extremely large amount of possible gene combinations that you can inherit from your father and mother. Plus the interaction of genes from your mother and father could cause characteristics that neither your mother nor your father had.

12. Mendel’s Concepts
There are alternate versions of genes and those are called alleles
For each characteristic, an organism inherits two alleles, one from each parent. If they are the same the organism is homozygous or is a homozygote. If they are different the organism is heterozygous or is a heterozygote.
In a heterozygote, the dominant allele determine the organism’s appearance and the recessive allele has no noticeable effect.
Law of Segregation- a sperm or egg carries only one allele for each characteristic because homologous pairs separate during meiosis.

13. Practice Problems

14. P, F1, F2 for plant height
P, F1, F2 for seed shape

15. In humans, free, or unattached, earlobes are dominant over attached earlobes. Suppose a man with the dominant phenotype for earlobes knows that he is homozygous for the trait, and his wife has the recessive phenotype. What are the possible genotypes? What are the possible phenotypes?
Suppose two individuals who are heterozygous for earlobe attachment have a child. What are the possible genotypes? What are the possible phenotypes?

16. Heterozygous yellow pea X green pea
Homozygous pea plant with axial buds X plant with terminal buds
Heterozygous pea plant with inflated pods is self pollinated
Purebred plant with yellow peas X heterozygous yellow pea
Hybrid pea plant X homozygous tall pea plant

17. Testcross
A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual.
A testcross can show whether the unknown genotype includes a recessive allele.
Mendel used testcrosses to verify that he had true-breeding genotypes.
The following figure demonstrates how a testcross can be performed to determine the genotype of a Lab with normal eyes.
Student Misconceptions and Concerns
Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. We would expect that any six playing cards dealt might be half black and half red, but we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling will reflect expected ratios.
Teaching Tips
Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. (If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three dark and one brown or all dark. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype.)
© 2012 Pearson Education, Inc. 17

18. What is the genotype of the black dog?
Testcross 
Genotypes
B_? bb
Two possibilities for the black dog: BB or Bb
Figure 9.6 Using a testcross to determine genotype
Gametes B B b b Bb b Bb bb
Offspring
All black
1 black : 1 chocolate
Figure 9.6 18

19. Yellow peas are dominant over green peas
You have a pea plant with yellow peas.
How could you determine whether your pea plant is homozygous or heterozygous?

20. Assume that in sheep white wool is dominant over the darker colored wool and that white wool is more valuable because it takes a particular type of dye.
A shepherd who has a small flock of various dark and white sheep wants to build a flock of homozygous white sheep, so he looks to by a healthy homozygous white ram.
A dealer assures him that his ram is homozygous white.
The shepherd agrees to buy the ram on the condition that he can return it the next year for a full refund if he has proof that the ram is heterozygous.
What can the shepherd do to discover the ram’s genotype?