Mendelian Patterns of Inheritance
Gregor Mendel was an Austrian monk who formulated some of the fundamental principles regarding the inheritance of traits. Between 1856 and 1863 he performed thousands of experiments in which he cross-bred pea plants with dichotomous characteristics such as color (e.g., yellow or green). Several conclusions were drawn from his studies:
- The hereditary determinants are finite factors called genes.
- Each parent has a gene pair in each cell for each trait studied. If one crosses two pure lines, one which is homozygous
for the dominant trait and one that is homozygous for the recessive trait, the progeny will be heterozygous and have one dominant allele and one recessive allele. - One member of a given gene pair segregates into a gamete, and each gamete carries only one allele for each gene.
- During fertilization, gametes unite randomly, independent other genes.
The gene that determines whether multiple lipomas will form (referred to on page 6) illustrates a Mendelian pattern of inheritance.
In this case, the "L" allele that encodes for multiple lipomas is dominant over the "l" allele which does not cause lipomas.
With a dichotomous trait like this one can one predictions about the proportions of offspring by using a Punnett square which shows the four possible pairs of alleles that can occur in the offspring. In Scenario #1 above, the Punnett square demonstrates that only heterozygous gene pairs are possible, so all of the offspring will have multiple lipomas, since the lipoma allele is dominant.
Table 1: Punnett Square for Offspring of a Homozygous Dominant (LL) Mother and a Homozygous Recessive (ll) Father
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Father's Alleles |
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|
l |
l |
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Mother's Alleles |
L |
Ll |
Ll |
|
L |
Ll |
Ll |
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All of the children wil he hetorzygous (Ll) and have the dominant trait.
Table 2: Punnett Square for Offspring of a Heteroygous (Ll) Mother and a Homozygous Recessive (ll) Father
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Father's Alleles |
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|
l |
l |
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Mother's Alleles |
L |
Ll |
Ll |
|
l |
ll |
ll |
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In this case half of the offspring (on average) will be heterozygous and have multiple lipomas, and the other half will be homozygous recessive and be free of lipomas.
Table 3: Punnett Square for Gender
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Father's Sex Chromosomes |
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X |
Y |
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Mother's Sex Chromosomes |
X |
XX |
XY |
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X |
XX |
XY |
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The mother has XX sex chromosomes and the father has XY, so half of the offspring are predicted to be female, and half will be male.
Dominant versus Recessive Inheritance Patterns
Some disease are inherited, and the pattern of appearance within a family tree will depend on whether the faulty allele is dominant or recessive compared to the normal allele. For example, the allele for Huntington's disease is dominant. If a heterozygous (Hh) man with Huntington's disease and a normal woman (hh) have children, some of them (about half on average) will have the disease (individuals shown in red). With a dominant allele like this, the disease occurs fairly frequently in the family tree.
In contrast to Huntington's disease, cystic fibrosis is caused by a recessive allele, meaning that individuals who are heterozygous for the cystic fibrosis allele (shown as Cc below) will not manifest any signs or symptoms of cystic fibrosis. As a result, the cystic fibrosis allele can be passed along a family tree with only sporadic appearance of individuals who have signs and symptoms of cystic fibrosis because they are homozygous for the recessive allele (cc).
Huntington's Disease
Cystic Fibrosis
Galtonian Patterns of Inheritance
Mendel's studies focused on dichotomous traits in plants, such as the color of peas (green or yellow) and plant size (tall or dwarf), but many traits have continuous distributions, such as height, weight, and intelligence. Galton was a contemporary of Mendel's who studied the inheritance of continuous characteristics. The idea that characteristics might be blended or averaged occurred to him when he noted that very tall fathers tended to have sons shorter than themselves, and extremely short fathers tended to have sons taller than themselves. He referred to this as "regression to mediocrity," and he concluded that height doesn't follow the inheritance patterns of the dichotomous traits that Mendel studied and that the phenomenon of dominance didn't apply here.
Mendelian inheritance patterns predicted some diseases, but only a few, and Galtonian genetics was limited by the inability to predict outcomes. R. A. Fisher, a British statistician and evolutionary biologist, was able to reconcile these two patterns of inheritance by showing that the inheritance of quantitative traits can be reduced to Mendelian inheritance if multiple genes are involved. For example, suppose the average height in a population is 68", and height is determined by one gene with 3 possible alleles: H0 (which neither adds nor subtracts from the average); H+2 (which adds 2" to height), and H-2 (which subtracts 2" from height). Suppose also the the H0 allele is twice as common as the other two alleles in the population. If these are co-dominant alleles, a Punnett square would predict the following inheritance patterns distribution of heights:
Adapted from http://www.uic.edu/classes/bms/bms655/lesson11.html
Now, if instead of just one gene, there were two genes that determined height, and both of them had three possible alleles as described above, then a Punnett square would predict a distribution with more categories and finerdifference among the categories.
Adapted from http://www.uic.edu/classes/bms/bms655/lesson11.html
It is easy to imagine that if there were three or more genes that were also determinants of height, the distribution would increasingly conform to a Gaussian distribution. In fact, a similar model with three gene loci, each with three alleles looks very much like a bell-shaped distribution. As a result, continuously distributed characteristics are likely to be determined by a finite number of genes which have co-dominant alleles. And, once again, it is important to point out that many environmental factors are likely to interact with the genotype to produce the final phenotype.
Sex-Linked Inheritance
Earlier in this modules it was noted that X and Y chromosomes are physically different from one another in that the Y chromosome is much shorter, and the Y chromosome only has about nine gene loci that match those on the X chromosome. As a result, almost all of the alleles on a male's single X chromosome are expressed, since there is no alternative dominant allele to mask them. This results in a distinct inheritance pattern for traits that are encoded on the X chromosome. For example, there are many types of color blindness. All of these conditions are inherited, and most of them are "sex-linked" because they are caused by defective alleles carried on the X chromosome. Red-green color blindness is a fairly common, mild form of color blindness which can be found in about 6% of the male population; it is far less common in females. This form of color blindness is caused by a recessive allele, and the inheritance pattern is illustrated in the figure below.
The defective allele (c) is only carried on the X chromosome, as is the normal allele (C), which is dominant. A heterozygous female, as shown above, would have normal color vision, but she would be a "carrier" of the allele for red-green color blindness. Now suppose that she marries a man with normal red-green color vision (he only has the C allele), and they have children. On average, half of the offspring will inherit mom's defective X chromosome, and half will inherit the X chromosome with the normal allele. Dad can contribute either an X chromosome with a normal allele or a Y chromosome with no allele for color vision. The Punnett square illustrates the possible combinations of alleles that will occur in the offspring. As you can see, there are four possible results. Both males and females can inherit the allele for color blindness, but it will not be expressed in any of the females, because the normal allele is dominant, so the heterozygous "Cc" females will be carriers of the trait but have normal color vision. In contrast, the males who inherit the defective allele will be color blind, because the Y chromosome doesn't have an allele to oppose it.
