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Genetics

Genetics

Genetics, Probabilty and Chance
 

Terms to know before you begin:

1. Gene:

2. Allele:

3. Genotype:

4. Phenotype:

5. Dominant:

6. Recessive:

7. Homozygous:

8. Heterozygous:

9. Trait:

Note: We will be doing lots of genetics problems today (in groups). You will not need a calculator. Read the notes and define the terms above, but don't worry about doing the problems until we are in class. --------------------------------------------------------------------------------

I. Overview: How are Traits passed from Generation to Generation? Genetics is the study of heredity - traits inherited from parent to offspring. Blending theory In ~1850, scientists thought that some fluid substance in the blood of animals or in the sap of plants was the hereditary material. The combination of the parent's characteristics in the offspring was thought to occur by a "blending" of this fluid. If so, a white dog that mated with a brown dog should produce only tan puppies; A tall person who had a child with a short person should produce all "medium-size" children, etc...clearly not the case! Even though people recognized problems with this theory, it was the top theory of the day! Keep in mind, though, that in the mid-1800s, very little was known about cell structure, let alone the concepts of genes and DNA...! A different theory was put forth by Gregor Mendel in 1850. Mendel was an Austrian monk who was interested in plant breeding. He performed careful experiments with the garden pea, Pisum sativum, collected large amounts of data, and in doing so, was able to uncover the basic principles of genetic inheritance that still hold true today! Mendel discovered that traits are interited in discrete units (we now know these to be genes). Mendel's discoveries were not understood by other scientists for over 35 years! --------------------------------------------------------------------------------

II. Mendel's experiments with the Garden Pea Mendel's work started when he bred two types of pea plants - ones with purple flowers and ones with white flowers - that were true-breeding for flower color (meaning that the purple flowers produced only plants with purple flowers and the white plants produced only plants with white flowers). Mendel cross-pollinated the flowers (pea plants usually are self-fertile). In the first filial (F1) generation, the white trait was masked (Note that a light purple "blended" color was NOT observed. Mendel took this observation one step forther, by allowing the F1 to "self". Result: The white trait re-appears in the F2 generation in a ratio of 3 purple plants to 1 white. Mendel did this experiment with a total of 7 different traits, studying 22 strains of peas and always using large sample sizes, and he always saw a ~3:1 ratio in the F2 generation (not shown = flower position on stem) --------------------------------------------------------------------------------

III. Mendel thought about how to explain what he saw: Mendel realized that these results were explainable if three things were true. He hypothesized that:

1. Every trait (like flower color, or seed shape, or seed color) is controlled by two "heritable factors". [We know now that these are genes - we each have two copies of every gene].

2. If the two alleles differ, one is dominant (will be observed in the organisms appearance or physiology) and one is recessive (cannot be observed unless the individual has two copies of the recessive allele). Dominant traits mask the appearance of recessive traits.

3. Alleles are randomly donated from parents to offspring - the factors (alleles) separate when the gametes are formed by meiosis, allowing all possible combinations of factors to occur in the gametes. Mendel's Law of Segregation - The two factors (alleles) separate when the gametes are formed, and only one factor (allele) is present in each gamete. --------------------------------------------------------------------------------

IV. Doing a genetic cross (monohybrid = 1 gene): Geneticists use letters be used to represent alleles. A capital letter = Dominant trait, a lowercase letter = a recessive trait. The same letter is used to indicate both alleles.

 Examples = Flower color: P= purple, p= white = Seed color: Y= yellow, y = green = Seed shape: W = wrinkled, w = round In humans... = Widow's peak: W = widow's peak, w = continuous hairline (which are you?) = Freckles: F = Freckles, f = no freckles (which are you?) = Earlobes: E = unattached, e = attached (which are you?) = Cystic fibrosis C = no CF, c = cystic fibrosis E-Z steps for doing genetics problems:

1. Indicate the genotype of the parents using letters

2. Determine what the possible gametes are

3. Determine the genotype and phenotype of the children after reproduction. To consider every type of offspring possible, use a Punnett Square in which all possible types of sperm are lined up vertically and all types of eggs are lined up horizontally:

4. Fill in the squares by "multiplying" the alleles from mom and dad: --------------------------------------------------------------------------------

Genetics Problem 1: (a) A man with a widow's peak (WW) marries a woman with a continuous hairline (ww). A widow's peak is dominant over a continuous hairline. What kind of hairline will their children have?

1. P1 Widow's peak (WW) x continuous hairline (ww)

2. Gametes: Male: W only, Female: w only 3. Children: (the F1 generation):

Genotype: Ww (all children will be heterozygous)

Phenotype: Widow's peak (phenotype of all children) --------------------------------------------------------------------------------

(b) Suppose one of their children (Ww) marries someone who is also heterozygous (Ww). What type of hairline will their children have? 1. P1 Widow's peak (Ww) x Widow's peak (Ww)

2. Gametes: Male: W and w; Female: W and w 3. Children: Genotype:

Their children have a 25% (1/4) chance of being WW, a 50% (2/4) chance of being Ww, and a 25% (1/4) chance of being ww. (Note that this is a 1:2:1 genotypic ratio IF both parents were heterozyhous to begin with) Phenotype: Their children will have a 3/4 chance of having a widow's peak and a 1/4 chance of having a continuous hairline (3:1 phenotypic ratio) --------------------------------------------------------------------------------

With small numbers of offspring (like humans vs pea plants) remember that these numbers represent the child's chance of having that trait. Each individual child has the same chance to inherit a trait from their parents.

Genetics problem 2: A man and a woman are heterozygous for freckles. Freckles (F) are dominant over no freckles (f). What are the chances that their children will have freckles? --------------------------------------------------------------------------------

Genetics problem 3: A woman is homozygous dominant for short fingers (SS). She marries a man who is heterozygous for short fingers (Ss). Will any of their children have long fingers (ss)? yes / no

Could any of their grandchildren potentially have long fingers? y / n Why or why not? --------------------------------------------------------------------------------

Genetics problem 4: Jane and John are expecting a baby and know that they are both carriers (ie heterozygous) of cystic fibrosis (Cc). What is the probability that their child will have cystic fibrosis (cc)?

What is the probability that their child will be a carrier of cystic fibrosis?

Chance of child being: _______ %

Disease free Genotype: ___________ _______ %

Cystic fibrosis carrier Genotype: ___________ _______ % Cystic fibrosis Genotype: ___________

Remember that this is only a probability, and the same probability occurs with each pregnancy!

http://www.biology.iupui.edu/biocourses/N100/2k4ch10genetics.html

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