Genetics - Patterns of Inheritance
March 20, 2000
Readings: Starr text: Ch 10 cover page, 10.1 - 10.8
"Knowledge is a sacred cow, and my problem is how we can milk her while keeping clear of her horns."
- Albert Szent-Györgyi, 1964
Please define these terms before you begin:
1. Gene: A unit of information about heritable trait that is passed on from parents to offspring. Each gene has a specific location (locus) on a chromosome.
2. Allele: At a given gene locus (location) on a chromosome, one of two or more slightly different molecular forms of a gene that arise through mutation and that code for different versions of the same trait.
3. Phenotype: Observable traits of an individual that arise from gene expression, gene interactions, and gene-environment interactions.
4. Genotype: the genetic makeup of an individual.
5. Monohybrid cross: An experimental cross in which offspring inherit a pair of nonidentical alleles for a single trait being studied, so that they are heterozygous.
6. Homozygous dominant: For a specified trait, having a pair of identical alleles at a gene locus (on a pair of homologous chromosomes) that are expressed regardless of the alleles with which they are paired. (ex: AA)
7. Heterozygous: Of a specified trait, having a pair of nonidentical alleles at a gene locus on a pair of homologous chromosomes). The dominant allele is expressed, and the recessive allele is masked. (ex: Aa)
8. Homozygous recessive: For a specified trait, having a pair of identical alleles at a gene locus (on a pair of homologous chromosomes) that are expressed alleles that can be masked by other alleles (recessive alleles). (ex: aa)
9. Trait: A heritable variation an individual's phenotype, which is determined by one (or more) genes.
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.
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'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 (can 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).
- 3. 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: you too can be a geneticist!
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.
- = Flower color: P= purple, p= white
- = Seed color: Y= yellow, y = green
- = Seed shape: W = wrinkled, w = round
- = 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: W, w
3. Children: (the F1 generation)
w w W W
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 W and w W and w
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? 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!
Dihybrid Cross (Note the characteristic 9:3:3:1 ratio).
Mendel also did a type of cross where two traits were followed at one time - a Dihybrid Cross.
However, one really important thing that Mendel noticed from this type of cross was that traits (like flower color, height) are inherited independently - not together as a unit. This has become known as
Mendel's Law of Independent Assortment - Genes for various traits assort into gametes independently (due to homologues lining up randomly at the metaphase plate).
II. Beyond Mendel's Laws
We know today that there are many exceptions to Mendel's laws. (Does this mean that Mendel was "wrong"? NO, it means that we know more today about disesase, genes, and heredity than we did 150 years ago!)
1. Incomplete dominance / Codominance: (example: blood type)
We keep discussing genes with only two alleles - but it is possible for genes to have three or more alleles (no person can have more than two however - one from Mom, one from Dad!)
One example of a three-allele gene: the ABO blood types of humans: (Fig 9.11)
- IA : Allele for type A blood
- IB : Allele for type B blood
- i : Allele for type O blood
- If you have Type A blood, you are either IA IA or IA i
- If you have Type B blood, you are either IB IB or IB i
- If you have Type O blood, you are ii (only choice!)
- If you have Type AB blood, you are IA IB (only choice!)
Note that the IA and IB alleles two alleles are both dominant to i - but one isn't dominant over another. This is called Incomplete Dominance or Codominance
Real life question: Kathy and Jim both have type A blood. Their daughter Julia has type O blood.
(a) What blood type alleles do both Kathy and Jim have?? ________________
(b) Their son Ian has never had his blood typed. What are the possible blood type alleles he might have? ____________________
Knowing blood types of the parents and the baby can aid in paternity suits.
2. Pleiotropy: One pleiotropic gene affects several different phenotypes. (example: sickle cell disease)
Example: Sickle cell disease - one defect in the hemoglobin gene - a single amino acid change in the protein - has many widespread and devistating effects
Another example: Marfan's syndrome - individuals have defects in skeletal system, eyes, and cardiovascular system. All these defects are due to one underlying defect - the body's ability to make connective tissue - one defect has widespread implications in the body
Abraham Lincoln had Marfan's syndrome.
3. Epistatic ("covering up") genes : An epistatic gene interferes with the expression of another gene (example: Laborador Retreivers)
Another example - Albinism in humans. Individuals with this trait lack pigment in all parts of their body. If a person inherits a double dose of the allele for albinism (aa), they will be unable to produce the pigment melanin, regardless what color their alleles for eye, skin, and hair color are.
4. Continuous Variation: Mendel studied "either-or" traits (purple vs white), but many characters such as human height and skin color vary as a continuum in populations (bell shaped curve)
a. Modifier genes: (example: eye color) One gene specifies B = brown or b = blue eyes. However, shades of grey and green eyes come from the action of modifier genes that control how much pigment is deposited in the iris of the eye.
Cool eye color website
b. Polygenic Inheritance: (example: human height) more than one gene controls a particular phenotype
In polygenic inheritance, more than one gene controls a particular phenotype. ie skin color is thought to be controlled by 3 alleles (A, B, C).
A person with the alleles AABBCC is very dark skinned, a person with aabbcc alleles is very light skinned, and a person with AaBbCc (or any combo) has an intermediate skin color.
Different "units" produce different shades (AAbbCc, aaBbcC, etc...)
Take-home quiz / practice genetics problems (handout in class) - Due Monday 3 / 27 (worth 4 extra credit points - and good practice!)