Chapter 12 - Inheritance
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I. Gregor Mendel and the Foundation for Modern Genetics
A. Gregor Mendel
1. A monk at a monestary in Brünn (now part of the Czech Republic)
2. Prior to settling into the monestary Mendel studied botany and mathematics at the University of Vienna
3. Mendel was not only a man of the cloth, but also a scientist who carried out a set of groundbreaking experiments that helped to pave the way for modern genetics.
B. Terminology
1. Locus – the physical location of a gene on a chromosome (plural: loci)
2. Homologous chromosomes carry the same genes at the same location (same locus)
3. Homologous genes do not have to be identical, but they are similar. These differing forms of the same gene are called alleles
4. If homologous chromosomes have the same allele at a particular locus, then the organism is termed homozygous for that locus.
5. If homologous chromosomes have differing alleles at a particular locus, then the organism is termed heterozygous for that locus.
6. Sometimes the terms “true breeding” and “hybrid” are used to describe homozygous and heterozygous loci respectively.
7. Therefore, each haploid cell that comes out of meiosis has only one allele for each locus. If the original cell was homozygous, all of the daughter cells will have the same allele; but, if the original cell was heterozygous, each allele will be present in half of the daughter cell population.
C. Secrets to Mendel’s Success
1. Choosing the right organism to work with - Mendel chose garden pea plants
a. Many variety of true-breeding pea plants were available at the time, providing a variety of characteristics for Mendel to examine.
b. Pea flowers can be experimentally manipulated to allow distinct matings to occur.
2. Designing and performing the experiment correctly
a. Mendel focused on specific characteristics that varied in a simple one to one fashion (purple vs. white flowers)
b. Mendel only looked at one trait at a time.
3. Analyze the data properly
a. Mendel tracked his breedings for multiple generations, looking for consistent patterns.
b. Mendel looked at the traits in a statistical manner (a new thought in his day)
II. How are single traits inherited?
A. Experimental design – Mendel raised plants that were true-breeding (homozygous) for different forms (alleles) of a single traits. He cross-breed them and saved the resultant hybrid seeds to grow the following year.
B. Mendel’s cross with reference to flower color
1. Parent 1: White flower
2. Parent 2: Purple flower
3. Cross-breed these two, referred to as the Parental (P) generation
4. All first-generation offspring (F1) were purple
5. The F1 plants were allowed to self-fertilize to produce the next generation
6. The second-generation offspring (F2) were split 3:1 purple to white flowers.
7. The F2 plants self-fertilized and produced the following.
a. White F2 à White plants after that
b. Purple F2
i. 1/3 produced consistent Purple flowers
ii. 2/3 produced similar results to the F1 self-fertilizations (3:1 purple to white)
C. How did the disappearing trait of white flowers (remember there are no white F1 plants) return in F2 and remain true-breeding afterwards?
D. Interpretation of the data
1. We know that traits are governed by distinct physical units called genes, and in the case of the above data each color is governed by a specific allele (white and purple).
2. In eukaryotes with homologous pairs each diploid cell has two alleles for a given trait. Those that are true-breeding have the same allele on each chromosome while those that are hybrids will have differing alleles.
3. During meiosis the alleles separate so that each gamete possesses only one allele for a given trait (Mendel’s law of segregation)
4. Separation of alleles is random since the separation of homologous chromosomes is random.
5. When two differing alleles are present, the presence of one (the dominant) may mask the other (the recessive). The phenotype of the organism is governed by the dominant allele, but the recessive allele is still present.
E. Mendelian Genetics Notation
1. Dominant alleles are designated by capital letters (typically the first letter of the dominant phenotype). In the case of flower color the dominant allele is purple color, so we use P.
2. The recessive allele is given the lower case letter of the dominant notation, so white flower color would be p.
3. True-breeding plants would only possess the alleles for the color that they always produce so we have.
a. Purple true-breeding: PP
b. White true-breeding: pp
4. Each F1 plant receives an allele from each of the two parents meaning all F1 plants are Pp in genotype.
1. Since P is the dominant allele, all F1 plants are purple
2. The p (white allele) is still present in the genotype, even though the phenotype between the F1 and PP parentals are indistinguishable.
5. This leads to the following possibilities for offspring of self-fertilized F1
a. ¼ = PP (Purple, true-breeding)
b. ½ = Pp (Purple, hybrid)
c. ¼ = pp (White, true-breeding)
6. One can use a short-hand bookkeeping method called the Punnett Square to simplify the system.
F. Mendel’s work led to the ability to predict results for crosses in a statistical fashion. Can you predict the outcome of a PP and Pp cross?
III. How are multiple traits inherited?
A. Mendel determined that traits that he studied were separated independently of each other. In other words, the outcome of a cross for seed color did not influence the outcome for seed shape.
B. Mendel was lucky (another trait of great science) in that all of his traits were found on separate chromosomes in the pea plant. Even in the original garden journal he kept the near perfect ratios can be found.
C. With what we know now we can state the law of independent assortment, which says that alleles of one gene will be separated independently of alleles for another gene as long as the two genes reside on different homologous pairs of chromosomes.
D. Sadly, though Mendel’s work was published in 1865 in the scientific literature it was not recognized until 1900 when three other scientist replicated the work… only to discover that Mendel had published the same findings 35 years earlier!
IV. How are genes located on the same chromosome inherited?
A. Genes on the same chromosomes tend to be inherited together.
1. Genetic linkage is the inheritance of certain genes as a group because they are on the same chromosome.
2. Interestingly the first pair of linked genes were discovered in sweet peas (a related but distinct plant from Mendel’s garden pea).
B. Recombination can create new combinations of linked alleles.
1. The linkage between alleles is inversely proportional to the distance separating the genes on the chromosome.
2. Recombination during Prophase I of meiosis can result in previously linked genes becoming unlinked and paired with different alleles.
V. How is sex determined, and how are sex-linked genes inherited?
A. In mammals and many insects males have a mismatched homologous pair of chromosomes, these are the sex chromosomes.
B. Females have homologous sex chromosomes (by convention females are XX, while males are XY)
C. A small portion of the sex chromosomes are homologous, so that they can pair during meiosis.
D. None sex chromosomes are referred to as autosomes.
E. Sex-linked genes are found only on the X or only on the Y chromosome.
1. Since females have homologous sex chromosomes (XX), they have the possibility of being heterozygous or homozygous for a trait.
2. Males, having only one sex chromosome, fully express whatever gene is present on their lone sex chromosomes (XY).
3. Try to think of the implications this would have for dominant and recessive genes.
VI. Do Mendalian rules of inheritance apply to all traits?
A. Incomplete Dominance: The phenotype of heterozygotes is intermediate between the phenotypes of the homozygotes.
1. RR = Red Flower in snapdragons
2. R’R’ = White Flower color in snapdragons
3. R’R = Pink Flower color in snapdragons
B. Codominance = a single gene has multiple alleles that can have complex inheritance patterns.
1. Blood groups in humans have three alleles A, B, and o
2. A and B are dominant over o, but A and B are both expressed if present.
a. Ao = A-type blood
b. Bo = B-type blood
c. AB = AB-type blood
d. oo = O-type blood
C. Polygenic Inheritance = the interactions of two or more genes contribute to a single phenotype.
1. The greater the number of genes involved, the greater the number of possible phenotypes.
2. Many complex traits (i.e. eye color, hair color, height, etc.) in humans are governed by this type of inheritance.
D. Pleiotropy = a single gene with multiple phenotypic effects (i.e. the SRY gene that controls sexual development of males in humans)
E. Environmental influences = the environment an organism grows in can influence the phenotypes expressed, this is the basis of the nature vs. nurture argument.
F. Remember, Mendalian ratios are statistical, human generation times are too long and each couple has too few offspring to make them directly applicable to a particular offspring.
VII. How are Human Genetic Disorders Investigated?
A. Human genetic crosses are ethically possible, so human geneticists have to indirectly acquire data about the genetics of a family.
B. A Pedigree is a family history that shows genetic histories for a trait.
C. These pedigrees can assist genetic counselors in advising a client as to the risk of a specific condition
D. Combined with the recent advances in molecular genetics many genetic disorders are being proactively addressed.
IIX. How are human disorders caused by single genes inherited?
A. Some human genetic disorders are caused by recessive alleles.
1. Heterozygous individuals are carriers
2. Heterozygous individuals are phenotypically indistinguishable from homozygous dominant individuals.
3. Albinism is the result of a recessive gene in melanin production
4. Sickle cell anemia is the result of a recessive gene resulting in abnormal hemoglobin.
B. Some human genetic disorders are caused by dominant alleles
1. Marfan’s syndrome
C. Some human genetic disorders are sex-linked
1. Since males only one sex chromosomes they tend to express sex-linked traits more frequently, even if they are recessive.
2. Red-Green Color-blindness
3. Hemophilia
D. Interestingly some heterozygous states can be advantageous.
1. Heterozygous individuals for sickle-cell anemia have increased resistance to the parasite that causes malaria (the 4th leading cause of death in the world)
2. Cystic Fibrosis heterozygotes seem to have given some form of resistance to bubonic plague in the Black Death
3. Genes of this type can be tracked by their greater than expected persistence in the population.
IX. Human disorders caused by improper chromosome numbers.
A. Nondisjunction is the improper partitioning of chromosomes during meiosis, resulting in gametes with too few and too many chromosomes.
B. Typically embryos with improper numbers of chromosomes spontaneously abort (accounting for 20-50% of miscarriages)
C. Most common nondisjunction mutations are in the sex chromosomes.
1. Turner Syndrome (XO) – Phenotypically female, but only have one sex chromosome, about 1:3000 women
a. Lack of sexual development
b. Short stature
c. Cardiovascular disease
d. Kidney defects
e. Hearing loss
f. Higher prevalence of sex-linked recessive phenotypes
2. Trisomy X (XXX) – female, about 1:1000 women
a. Most have no detectable defects
b. Slight tendency for increased height and below-normal intelligence.
c. Fertile, with normal children.
3. Klienfelter Syndrome (XXY) – male, about 1:1000 men
a. Most with no detectable defects
b. At puberty, some show mixed characteristics (enlarged breasts, broadening hips, and small testes)
c. Typically infertile due to low sperm count.
4. XYY Males – male, 1:1000 men
a. Severe acne
b. High testosterone
c. Taller than average (2/3 over 6 ft., as opposed to 5’9” average)
d. Most have little phenotypic signs
e. A current topic is whether XYY males are more prone to violence; this assumption rides on questionable statistics and has not been conclusively shown.
D. Autosomal Defects
1. Trisomy 21 (Down Syndrome) – 1:900 births
a. Distinct physical characteristics
b. Increased susceptibility to infectious disease
c. Heart Malformation
d. Varying degree of mental retardation
e. Prevalence increases with the age of the parents, especially the mother.