Introduction to Mendelian Genetics In The Simplest Terms

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

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

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.

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.

• 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.

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.

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)

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.

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

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

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.

Let’s take it one step further… • Let’s say that two of the blue hairs from our F 1 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. B • Let’s set up another square to see what type of offspring they might have: b • 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 Bb bb

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

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

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.

Here are some sample problems:

1. On planet X, blue hair is dominant to yellow hair. A heterozygous blue hair mates with a yellow hair. • 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 b b Bb bb

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.

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

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 • homozygous black x white b Bb Bb • two heterozygous blacks • heterozygous black x white B B b b Bb bb B BB Bb bb

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

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 wrinkled

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

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

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 b Bb bb

Monohybrid Cross Worksheets Frog Book

BB Bb Bb bb b B Bb b Bb Bb b B Bb b bb b Bb bb 25 50 25 75 25 0 100 0 0 50 50

Dihybrid Cross Frog Worksheet

b. E be Bb. Ee. Bbee bb. Ee bbee To find the possible gamete combinations use FOIL 25 Bb. Ee BE Be b. E be BE BBEe Bb. EE Bb. Ee Be BBEe BBee Bb. Ee Bbee b. E Bb. Ee bb. EE bb. Ee be Bb. Ee Bbee bb. Ee bbee 9 3 3 1 6. 25