1. Agenda 4/9 Mendelian Genetics Lecture Mendelian Genetics Practice
Homework: Chp 14 notes (due Wednesday), genetics video and notes, genetics worksheet
Warm Up: Explain one trait that is NOT fully a genetic trait.

2. Genetics: Part I
Mendel and the Gene

3. Colorblindness Polydactyly Cleft chin Marfans Syndrome Freckles
Our traits make us who were are. The expression of your genetic code determines how you look and function. In this unit we will review simple genetic expression as well as take a look at more complex patterns of inheritance.
Marfans Syndrome
Freckles
Widows peak

4. Mendelian Genetics
What do you remember about Mendel and his genetics studies from your first biology course? What are some of the terms of genetics that you remember?
What do you remember about Mendel and his genetics studies? (Austrian monk who studied traits in pea plants)
Work with a partner to generate a list of terms that you remember or think of when you think about Mendelian genetics.

5. Can you define these? Genotype Heterozygous Phenotype Allele
Monohybrid
Test cross
Dominant F1
Recessive F2 P1
Homozygous
Pair up with someone near you to describe what you remember about each of these terms. (allow them time to give explanation and descriptions) We will come back to this list later.

6. An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character
An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character
Unlike homozygotes, heterozygotes are not true-breeding

7. Explain three benefits to using plants for genetic research

8. PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous)
Phenotype
Genotype
Purple
PP (homozygous) 1 3
Pp (heterozygous)
Purple 2
Pp (heterozygous)
Purple
Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition
Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup
In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes
pp (homozygous) 1
White 1
Ratio 3:1
Ratio 1:2:1

10. Mendel chose to track only those characters that occurred in two distinct alternative forms
He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)

11. P
For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers
These alternative versions of a gene are now called alleles
Each gene resides at a specific locus on a specific chromosome
For each character, an organism inherits two alleles, one from each parent
Mendel made this deduction without knowing about the role of chromosomes
The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation
Alternatively, the two alleles at a locus may differ, as in the F1 hybrids
If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant p

12. Mendelian Genetics
Law of Dominance - if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
Lets begin by looking at some of the fundamentals of genetics as presented by Mendel, starting with the law of dominance. When Mendel crossed F1 green pod peas, he noted yellow pods in the F2 generation. The green allele had masked the expression of the yellow pod color in the F1. The yellow pod color could be expressed in the F2 generation when the recessive allele from one parent combined with the that from the other parent. Dominant alleles are those that mask the appearance of other alleles.

13. Human Traits with simple dominance:
Widows peak
Bent little finger
Handedness
Nearsightedness
Free earlobes
Mid-digit hair
Cleft Chin
Dimples
Freckles
Polydactyly
Some human traits are thought to be controlled by single alleles expressing dominance.

14. Dominant does not mean numerous
Frequency of Dominant Alleles
Dominant alleles are not necessarily more common in populations than recessive alleles
For example, one baby out of 400 in the United States is born with extra fingers or toes
The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage
In this example, the recessive allele is far more prevalent than the population’s dominant allele

15. Dark hair alleles are equally common in all parts of Europe.
In humans, alleles for dark hair are genetically dominant, while alleles for light hair are recessive. Which of the following statements is/are most likely to be correct?
Dark hair alleles are more common than light hair alleles in all areas of Europe.
Dark hair alleles are more common than light hair alleles in southern Europe but not in northern Europe.
Dark hair alleles are equally common in all parts of Europe.
Dark hair is dominant to light hair in southern Europe but recessive to light hair in northern Europe.
Answer: b
This question confronts a widespread misconception. Many people think (incorrectly) that the genetically dominant allele will be most frequent and that dominant alleles increase in frequency within populations over time. The Hardy-Weinberg law shows that this is not the case; dominance and frequency are not related. Allele frequency of dominant alleles can be low (dark hair color in northern Europe) or high (dark hair color in southern Europe). Answer b is the only answer that reflects this pattern. Even though the chapter does not address population genetic ideas, it’s a good idea to try to break this misconception early.

16. Mendelian Genetics
Law of segregation: the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
The law of segregation explains that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
Thus, an egg or a sperm gets only one of the two alleles that are present in the organism.
Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses
The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup
A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele
This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis
Segregation leads to variation in gametes. Ultimately giving variation to the offspring. Variation is one of the primary advantages of sexual reproduction. It is this variation that provides adaptability within the offspring.

17. Dominant phenotype, unknown genotype: PP or Pp?
Figure 14.7
TECHNIQUE
Dominant phenotype, unknown genotype: PP or Pp?
Recessive phenotype, known genotype: pp
Predictions
If purple-flowered parent is PP or
If purple-flowered parent is Pp
Sperm
Sperm p p p p P P Pp Pp Pp Pp
Eggs
Eggs
How can we tell the genotype of an individual with the dominant phenotype?
Such an individual could be either homozygous dominant or heterozygous
The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual
If any offspring display the recessive phenotype, the mystery parent must be heterozygous P p Pp Pp pp pp
RESULTS or
All offspring purple
1/2 offspring purple and 1/2 offspring white

18. Hypothesis of dependent assortment
Figure 14.8
EXPERIMENT
YYRR
P Generation
yyrr
Gametes YR yr
F1 Generation
YyRr
Predictions
Hypothesis of dependent assortment
Hypothesis of independent assortment
Sperm
Predicted offspring of F2 generation or
1/4 YR
1/4 Yr
1/4 yR
1/4 yr
Sperm
1/2 YR
1/2 yr
1/4 YR
YYRR
YYRr
YyRR
YyRr
1/2 YR
YYRR
YyRr
1/4 Yr
Eggs
YYRr
YYrr
YyRr
Yyrr
Eggs
Mendel identified his second law of inheritance by following two characters at the same time
Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently
1/2 yr
YyRr
yyrr
1/4 yR
YyRR
YyRr
yyRR
yyRr
3/4
1/4
1/4 yr
Phenotypic ratio 3:1
YyRr
Yyrr
yyRr
yyrr
9/16
3/16
3/16
1/16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101 32
Phenotypic ratio approximately 9:3:3:1

19. Independent Assortment
Using a dihybrid cross, Mendel developed the law of independent assortment
The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation
Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome
Genes located near each other on the same chromosome tend to be inherited together

20. Practice
In order to solidify your ability to explain the Laws of Segregation and Independent Assortment, describe how these laws would apply as sperm cells are developed in a man with genotype RrSs.
Draw an image to support your explanation.

21. Sample image
The alleles for each trait, separate from one another during segregation and will end up in different gametes. Notice that in this diagram the R is never in the same cell with the r. The S is never in the same cell with the s.
Independent assortment refers to the manner in which traits will sort without regard to other traits provided they are carried on different chromosomes. The four cells are drawn to show the various combinations that can occur.

22. Show the possible gametes for a mother with a genotype RRSs

23. a. two gamete types: white/white and purple/purple
Imagine crossing a pea heterozygous at the loci for flower color (white versus purple) and seed color (yellow versus green) with a second pea homozygous for flower color (white) and seed color (yellow). What types of gametes will the first pea produce?
a. two gamete types: white/white and purple/purple
b. two gamete types: white/yellow and purple/green
c. four gamete types: white/yellow, white/green, purple/yellow, purple/green
d. four gamete types: white/purple, yellow/green,white/white, and purple/purple
Answer: c
The purpose of this question is to help students figure out gamete types—a step they often rush past. Students need to realize that gametes are haploid and that each gamete contains one, and only one, allele for each of the traits being studied. The presence of the second pea in the question stem is a distracter. Answer a is wrong because it shows gametes with two alleles for flower color and no alleles for seed color. Answer b is wrong because it shows only two possible gamete types. Answer c is right because it shows all four possible gamete types. Answer d is wrong because it shows gametes that are diploid for one trait and containing an allele for only one locus.