1. Introduction to Mendelian Genetics
In The Simplest Terms I Can Think Of

2. All living things have observable traits
All living things have observable traits. We call these observable characteristics a phenotype.

3. Living things usually have two genes that determine each trait
Living things usually have two genes that determine each trait. This corresponds with the fact that most living things have two parents – or at least both male & female parts.

4. We represent these genes with letters – capital or lower case – but use the same letter for each trait. It doesn’t really matter which letter you use – as long as you use the same letter for each trait.

5. You will use an upper case letter if the trait is dominant (A)
You will use a lower case letter if the trait is recessive (a)
The upper & lower case forms of the letter represent different gene forms or alleles.

6. Remember, you have two genes for each trait so there are three possible combinations: AA or Aa or aa When we write the letters like this we call it a genotype.

7. Whenever the dominant gene is present in the genotype, the dominant phenotype is observed. (AA or Aa) The only way to observe a recessive phenotype is to have two recessive genes together in the genotype. (aa)

8. If you have two of the same “case” (upper or lower) in a genotype it is considered homozygous or pure. If you have two different “case” (upper & lower) in the genotype it is considered heterozygous or hybrid.

9. For Example: Let’s say that on planet X blue hair is dominant to yellow hair.
How did you know which was dominant or recessive? I had to tell you that.
So we need to assign some letters to the genes:
Again, it doesn’t matter what letter you use. A lot of times a letter is chosen based on the dominant trait.
B = blue b = yellow
DO NOT PUT A “Y” FOR YELLOW - YOU HAVE TO USE THE SAME LETTER FOR EACH DIFFERENT TRAIT!
So let’s say that a homozygous blue boy being mates with a homozygous yellow female.
First you have to think – what are their genotypes? homozygous blue = BB homozygous yellow = bb

10. Then we can make a Punnett square to demonstrate the cross in an easy to observe form.
Each parent will give one gene (letter) to a potential offspring
Put the female genes (letters) above each of the top squares. In this case she can give a “b” or “b”
Put the male genes (letters) along the left side. In this case he can give a “B” or “B”
Then fill in the boxes for the possible combinations b b B Bb Bb B Bb Bb

11. So there is only one possible genotype for this combination: All offspring (100% or 4/4) will be Bb. All (100% or 4/4) will have a “blue hair” phenotype since even though they are hybrid, they carry the dominant gene which will mask the recessive trait.

12. Let’s take it one step further…
Let’s say that two of the blue hairs from our F1 generation mate – because that is okay on planet X.
Both have the genotype Bb – they can give either a “B” or “b” to the potential offspring.
Let’s set up another square to see what type of offspring they might have:
In this case, there are three possible genotypes of offspring: 1/4 (25%) BB, 2/4 (50%) Bb, 1/4 (25%) bb
The phenotype ratios are: 3/4 blue (3 have a “B”) and 1/4 yellow. B b BB Bb B b Bb bb

13. Both of the above examples are referred to as a monohybrid cross-since we only studied one trait-hair color.

14. Here’s something you may have noticed: there are only two phenotypes, but three possible genotypes.

15. How could you determine if something showing a dominant trait is homozygous (BB) or heterozygous (Bb)?
A surefire method would be to cross the thing in question with a thing showing the recessive trait – because you know its genotype: bb – they are always homozygous recessive. This is called a TEST CROSS.
If you get all things showing a dominant trait, the organism was probably homozygous (BB) – and the offspring are heterozygous (Bb)
If you get any things showing the recessive trait (bb) it had to be a hybrid (Bb) since it had to have had a recessive gene to pass on the homozygous recessive offspring.

16. Here are some sample problems:

17. 1. On planet X, blue hair is dominant to yellow hair
1. On planet X, blue hair is dominant to yellow hair. A heterozygous blue hair mates with a yellow hair. B b
a. Give the genotypes for the two things being crossed:
Bb X bb
b. Make a Punnett square to show the cross.
c. Give the probabilities of the percentages of genotypes & phenotypes in offspring.
50% Bb, 50% bb; 50% blue, 50% yellow b Bb bb b Bb bb

18. 2. On planet X, a blue haired male mates with a yellow haired female
2. On planet X, a blue haired male mates with a yellow haired female. They have a child with yellow hair. What then, must the genotype of the father be?
The father must be Bb to have passed on a recessive gene but still show the dominant trait.

19. *both parents must carry a “b” to pass to offspring Bb x Bb bb x bb
3. On planet X, what 3 combinations of parent genotypes could have a child with yellow hair?
*both parents must carry a “b” to pass to offspring
Bb x Bb
bb x bb
Bb x bb

20. 4. In a certain animal, black fur (B) is dominant to white fur (b)
4. In a certain animal, black fur (B) is dominant to white fur (b). Determine the expected genotypic ratios & phenotypic ratios resulting from crosses between: B B
 homozygous black x white
 two heterozygous blacks
 heterozygous black x white b Bb Bb b Bb Bb B b B BB Bb Bb bb b B b b Bb bb b Bb bb

21. 5. Which of the following genotypes are heterozygous:AA Aa Yy yy Rr RR no √ √ no √ no

22. RR round Rr rr round wrinkled
6. Round (R) is dominant shape for seeds of pea plants. Wrinkled (r) is recessive. What are the phenotypes for the following allele pairs: RR Rr rr
round
round
wrinkled

23. 7. In a plant with genotype Rr for seed shape, what percentage of the gametes will have the
R allele?
r allele?
50%
50%

24. 8. If the gene for freckles (F) is dominant over the gene for no freckles (f), what are the possible genotypes of the parents of a child who does not have freckles?
For a child not to have freckles, both parents must have the “f” allele.
Ff x ff, ff x ff, Ff x Ff

25. We know the man is bb if he shows the recessive trait.
9. In human beings, brown eyes (B) are dominant over blue eyes (b). Suppose a blue eyed man marries a brown eyed woman whose father was blue eyed. What proportion of their children would you predict would have blue eyes?
We know the man is bb if he shows the recessive trait.
If the woman’s father had blue eyes he had to have passed a “b” to her so her genotype must be Bb
50% should have blue eyes B b Bb bb b Bb bb b

26. Monohybrid Cross Worksheets
Frog Book

27. 25 50 BB Bb 25 Bb bb 75 25 B B
100 b Bb Bb b Bb Bb
100 B b 50 b Bb bb 50 b Bb bb 50 50

28. Dihybrid Cross
Frog Worksheet

29. To find the possible gamete combinations use FOILbE be
BbEe
Bbee
bbEe
bbee be
BbEe
Bbee
bbEe
bbee be
BbEe
Bbee
bbEe
bbee 25 be
BbEe
Bbee
bbEe
bbee
BbEe
BbEe BE Be bE be BE
BBEE
BBEe
BbEE
BbEe Be
BBEe
BBee
BbEe
Bbee bE
BbEE
BbEe
bbEE
bbEe be
6.25
BbEe
Bbee
bbEe
bbee 9 3 3 1