Genetics- Part 1- Genes
Mendel
Mendel was an Austrian monk who taught natural science and worked on plant breeding experiments.
He developed a basic understanding of genetics and inheritance.
Mendel’s Work
It took him 2 years to select the pea plant as his subject.
He collected data for 10 years.
His sample sizes were large; he tabulated results from 28,000 pea plants.
He replicated his experiments.
He analyzed his data with statistics (probability theory).
Characteristics of Garden Peas:
Peas are easy to grow, and take little space.
They are inexpensive.
They have a short generation time compared to large animals so that a large number of offspring can be obtained in a short amount of time.
They have some distinct characteristics that are easy to recognize. These characteristics can be used when trying to determine patterns of inheritance.
They are easily self-fertilized or cross fertilized.
Traits Studied by Mendel
smooth or wrinkled seeds
yellow or green seeds
red or white flowers
inflated or constricted pods
green or yellow pods
axial or terminal flowers
tall or dwarf plants
Mendels Crosses
Mendel used pure-breeding individuals in the first (P1) generation.
P1 yellow X green
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F1 all yellow
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F2 3/4 yellow, 1/4 green
Conclusions from Mendel's Crosses
The F1 generation showed only one character that was present in the P1. The other character reappeared in the F2 (25%).
The sex of the parent did not matter.
The traits did not blend.
Mendel concluded that the F1 plants must contain 2 discrete factors, one for each character. The character that was seen in the F1 is called dominant. The character not seen in the F1 is called recessive.
Letters Can Represent Genes
The characteristics studied by Mendel were due to single genes. On the pair of chromosomes diagrammed below, the letter "A" represents a gene for yellow seeds. The letter "a" on the homologous chromosome represents a gene for green seeds. By convention, upper case letters are used to represent dominant genes and lower case letters are used for recessive genes.
Because individuals are diploid, two letters can be used to represent the genetic makeup of an individual. In the case of seed color, the following three gene combinations are possible: AA, Aa, and aa.
Heterozygote (also called hybrid) refers to an individual that has two different forms of the gene. Example: Aa
Homozygote refers to an individual that has two identical genes. Example: AA or aa
A hybrid is a heterozygote. Example: Aa
Principle of Segregation
Mendel’s principle of segregation states that paired factors (genes) separate during gamete formation (meiosis). Because the pair of genes (Aa, AA, or aa) separate, one daughter cell will contain one gene and the other will contain the other gene. (See diagram above.)
Gametes
Because pairs of chromosomes separate during meiosis I, gametes are haploid, that is, they carry only one copy of each chromosome. An Aa individual therefore produces two kinds of gametes: A and a.
Below: An "AA" individual produces all "A" gametes. Similarly, an "aa" individual produces all "a" gametes.
Individual (genotype) / Type of gametes producedAA / all gametes will contain an "A"
Aa / 1/2 will contain "A" and 1/2 will contain "a"
aa / all "a" gametes
Punnett Squares
Suppose that an "Aa" individual is crossed with another "Aa" individual. One will produce "A" eggs and "a" eggs. The other will produce "A" sperm and "a" sperm. What are all of the possible combinations of eggs and sperm? A Punnett square can be used to show all of these combinations.
The Punnett square in the diagram below is used to show between two Aa individuals.
The square below is used for this cross: AA X Aa.
One half of the offspring produced by this cross will be AA, the other half will be Aa.
The cross can also be written as shown below because the AA parent can produce only one kind of gamete (all A).
A Closer look at Mendel’s Crosses (One Gene Locus)
Y = yellow y = green
P1 YY X yy
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F1 Yy
Yy X Yy ¬ A cross between two individuals that are heterozygous for a trait is called a monohybrid cross.
F2 The above cross is illustrated below.
Genotype and Phenotype
The genetic makeup of P1 plants was different from that of F1 because the P1 plants were true breeding and the F1 plants were not. The genetic makeup of an individual is referred to as its genotype. Because the plants are diploid, two letters can be used to write the genotype. In this case, the genotype of the P1 plants was YY; the genotype of the F1 plants was Yy.
The characteristics of an individual are its phenotpye. This word refers to what the individual looks like so ddjectives are used to write the phenotype. For example, "yellow" or "tall" are phenotypes. The yellow P1 plants looked like the F1; they had the same phenotype but different genotypes.
An individual with a recessive phenotype has two recessive genes. A dominant phenotype results from either one or two dominant genes. In the cross above, YY or Yy are yellow; yy is green. The phenotype ratio in the F2 is 3 yellow:1 green. The genotype ratio is 1YY:2Yy:1yy.
Genotype / PhenotypeAA or Aa / Yellow
aa / Green
Other Crosses
S = smooth s = wrinkled
P1 SS X ss
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F1 Ss
Ss X Ss
F2 genotype ratio = 1:2:1 (1SS : 2Ss : 1ss)
phenotype ratio = 3:1 (3Smooth : 1 wrinkled)
F = full f = constricted
P1 FF X ff
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F1 Ff
Ff X Ff
F2 genotype ratio = 1:2:1 (1FF : 2Ff : 1ff)
phenotype ratio = 3:1 (3full: 1 constricted)
Alleles and Loci
An allele is a gene that has more than one form. Each of the forms is referred to as an allele. For example, the gene for red flowers and the gene for white flowers are two different alleles.
A locus (plural: loci) is the location of a gene on a chromosome. The gene for red flowers and the gene for white flowers are two different alleles at the same locus. A single chromosome can have a gene for white flowers or a gene for red flowers but not both.
There are two loci illustrated below, one is for flower color and the other is for stem length. Flower color has five alleles and stem length has two.
Application
Sickle-cell anemia is an abnormality of hemoglobin, the molecule that carries oxygen in our blood. Red blood cells of affected individuals often become distorted in shape, they then may break down or clog blood vessels causing pain, poor circulation, jaundice, anemia, internal hemorrhaging, low resistance, and damage to internal organs.
This condition is caused by a recessive gene.
A = normal hemoglobin
a = sickle-cell hemoglobin
AA = normal
Aa = normal (called sickle-cell trait)
aa = sickle-cell anemia
A man with sickle-cell trait marries a normal woman. What is the probability that their children will have sickle-cell trait?
If both parents have sickle-cell trait, what percentage of their children will:
have a normal phenotype?
have sickle-cell trait?
have sickle-cell anemia?
Testcross - One Locus
let A = red
a = white
Is a red flower AA or Aa?
Solution: cross it with aa
P1 A? X aa
The A? individual can produce these kinds of gametes: "A" and "?"
gametes: A, ? and a
F1 Aa and ?a
If the ?a individual is red, then ? = A. If it is white, then ? = a.
Should There Be Fewer Recessive Alleles?
The population model described above predicts that gene frequencies will not change from one generation to the next even if there are more recessive alleles.
There is sometimes a misconception among students beginning to study genetics that dominant traits are more common than recessive traits. Sometimes this is true, sometimes it is not. For some traits, the dominant is more common; for other traits, the recessive is more common. For example, blood type O is recessive and is the most common type of blood. Huntington's disease (a disease of the nervous system) is caused by a dominant gene and the normal gene is recessive. Fortunately, most people are recessive; the dominant is uncommon.
The misconception comes from the observation that in a cross of Aa X Aa, 3/4 of the offspring will show the dominant characteristic. However, the 3:1 ratio comes only if the parents are both Aa. If there are many recessive genes in a population, then most matings are likely to be aa X aa and most offspring will be aa.
In nature, natural selection may favor one- either the dominant or the recessive- and that one will become more common over time. Other forces such as genetic drift may also cause one or the other allele to become more common. In the absence of forces that change gene frequencies, there is no reason to expect dominant genes to be more common.
Probability
Multiplicative Rule
The probability of two or more independent events occurring is equal to the product of their probabilities.
Example: What is the probability of tossing a coin two times and getting a heads both times?
Solution: The probability of getting a heads on one coin is 1/2. The probability of getting a heads on the second coin does not depend on the outcome of the first coin, so the multiplicative rule is used. The probability of getting a heads on two coins is 1/2 X 1/2 = 1/4.
Additive Rule
The probability of two or more mutually exclusive events occurring is equal to the sum of their probabilities.
What is the probability that a student will get an “A” or a “B” in a class if students generally earn the following grades:
A = 10% (or 0.10)
B = 35% (or 0.35)
C = 45%
D = 10%
Solution: In this example, the two outcomes (getting an "A" or getting a "B") are mutually exclusive because you can only get one or the other. The additive rule is used to determine the overall probability of getting an "A" or a "B". 10% + 35% = 45% (or 0.10 + 0.35 = 0.45).
Consider Two Loci at the Same Time
Independent Assortment
Genes that are on different chromosomes assort independently. The following are four different metaphase I allignment patterns that are possible for a hypothetical species with a diploid chromosome number of 6.
The alignment pattern shown in the diagram below will produce Sy and sY gametes.
The alignment pattern shown in this diagram will produce SY and sy gametes.
Both of the patterns illustrated above are possible because S and Y are located on different chromosomes.
Possible Gametes for Several Different Genotypes
The table below shows the kinds of gametes that can be produced by several different kinds of genotypes. Each gene locus (A and B) is on a different chromosome.
Individual / GametesAABB / AB
AABb / AB, Ab
AaBB / AB, aB
AaBb / AB, Ab, aB, ab
Aabb / Ab, ab
AAbb / Ab
aaBB / aB
aaBb / aB, ab
aabb / ab
Genotypes and Phenotypes
let A = red, a = white
let B = smooth, b = wrinkled
The table below shows possible genotypes and phenotypes.
Genotype / PhenotypeAABB / red, smooth
AABb / red, smooth
AaBB / red, smooth
AaBb / red, smooth
Aabb / red, wrinkled
AAbb / red, wrinkled
aaBB / white, smooth
aaBb / white, smooth
aabb / white, wrinkled
Linkage
In peas, the locus for seed texture (smooth or wrinkled) and seed color (yellow or green) are on two different chromosomes so they assort independently.
Suppose that they are on the same chromosome as indicated in the diagram below. Independent assortment will not occur because the "S" gene is on the same chromosome as the "y" gene. Similarly, the "s" gene is on the same chromosome as the "Y" gene. Unless crossing-over occurs, "S" will always be found with a "y" and "s" will be found if there is a "Y".
Mendel studied seven different characteristics in peas. Each of these characteristics are on different chromosomes, so they assort independently.
Example: Two Gene Loci
Let S = smooth, s = wrinkled
Let Y = yellow, y = green
P1 SMOOTH, YELLOW X wrinkled, green
genotypes: SSYY ssyy
gametes: SY sy
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F1 SMOOTH, YELLOW X SMOOTH YELLOW
genotypes: SsYy X SsYy ¬ si icol eneg owt rof suogyzoreteh era taht slaudividni owt neewteb ssorc A a dellac ssorc dirbyhid.
gametes: SY, Sy, sY, sy
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F2
Mendel's Results
SMOOTH, YELLOW / 315SMOOTH, green / 108
wrinkled, YELLOW / 101
wrinkled, green / 32
556
A general rule for dihybrid crosses (AaBb X AaBb)
TRAIT 1, TRAIT 2 X trait 1, trait 2 (upper case traits are dominant)
9 - TRAIT 1 and TRAIT 2 expressed (A-B-)
3 - TRAIT 1 expressed (A-bb)
3 - TRAIT 2 expressed (aaB-)
1 - No dominant traits expressed (all aabb)
A dihybrid cross is two monohybrid crosses
Remember that each of the individual traits in the dihybrid cross above behaves as a monohybrid cross, that is, they will produce a 3:1 phenotype ratio in the offspring.
SMOOTH X wrinkled
Refer to the F2 data for the SMOOTH, YELLOW X wrinkled, green cross above.
The number of smooth offspring was 315 + 108 = 423.
The number of wrinkled was 101 + 32 = 133.
The ratio of smooth to wrinkled is therefore 423:133 or approximately 3:1.
YELLOW X green
yellow = 315 + 101 = 416
green = 108 + 32 = 140
ratio = 416:140 or approximately 3:1
Combining Probabilities