The last lecture on mitosis and meiosis discussed how cells reproduce. Mitosis and meiosis both involve the passing on of chromosomes from parents to offspring -- we made the point that chromosomes carry the instructions that determine the characteristics of the cell.
The next two lectures will consider in more detail how biological characteristics of organisms are passed from parent to offspring -- the study of the inheritance of biological traits is the area of biology known as genetics.
First, a bit of history -- in the mid-1800's the prevailing view was that the hereditary material was a fluid, perhaps blood -- terms still in use that reflect this idea: bloodline, blue blood, etc. -- inheritance was viewed as the mixing of blending of fluids every generation = theory of blending inheritance.
This concept of inheritance was challenged by an Austrian monk named Gregor Mendel -- Mendel meticulously carried out breeding experiments in the monastery garden using the garden pea -- he published his work around 1860 and it was completely ignored by the scientific community -- rediscovered in 1900 and its importance was immediately recognized -- Mendel is generally recognized as the "father of modern genetics."
Let's begin by looking at the kinds of experiments Mendel did, the kinds of results he got, and the basic genetic principles that emerged from these results.
1. Mendel analyzed patterns of inheritance for seven separate traits each having two characteristics -- e.g. seed color (yellow, green); seed coat texture (smooth, wrinkled); plant height (tall, short), etc.
2. Mendel first made sure that his parental plants (P1 generation) bred true -- crossing (selfing) members of one parental type (e.g. purple flowers) always produced progeny that had the same characteristics (e.g. purple flowers).
3. Mendel then began crossing contrasting parental plants in the P1 -- e.g. plants with purple flowers X plants with white flowers -- found that the next generation (F1) was always made up entirely of individuals having the characteristic of only one of the parental lines -- e.g. F1 were purple flowers.
4. Found that if the F1 was allowed to self to generate an F2, the parental characteristic missing in the F1 reappears, but the characteristic present in the F1 outnumbers the other by a ratio of 3:1 -- this ratio is important.
5. Repeating this experiment with each of his 7 traits, Mendel invariably observed an F2 ratio of 3:1.
Several important points emerge from these experiments which Mendel recognized:
1. dominance-recessiveness: disappearance of one parental characteristic in the F1 and its reappearance in the F2 convinced Mendel that both traits were present in the F1 but one (the dominant) was preventing the other (the recessive) from being expressed -- we now know that many, though not all traits have dominant and recessive patterns of inheritance.
2. Mendel's law of segregation: an organism contains two discrete hereditary factors for each trait and these factors segregate (separate) during gamete formation so that each gamete contains only one factor from each pair of factors.
These two concepts provided Mendel with a tentative explanation for the results of his experiments.
Law of Segregation -- challenged the idea of blending inheritance -- Mendel viewed his discrete factors as being particulate in nature -- particulate inheritance -- now know that chromosomes and their behavior during meiosis are the mechanism underlying this law -- should note that the existence of chromosomes and the details of meiosis were unknown to Mendel.
Now, lets introduce some modern genetics terminology to Mendel’s experiments:
1. genetic traits such as the ones Mendel studied are controlled by a single gene = segment of a DNA molecule containing the instructions of how a single polypeptide is to be constructed -- e.g. gene for flower color in peas -- not all traits are controlled by single genes, but many are (more later).
2. a given gene may carry alternate messages -- alternate forms of a gene are called alleles -- alleles are given a symbolic designation and may be referred to as "dominant" or "recessive", if appropriate -- e.g. the purple allele for flower color in peas is the dominant allele and is designated A, while the allele for white flowers is recessive and is designated as a.
3. now, each individual carries two genes for a trait (Mendel's law of segregation) -- one gene is carried on each member of a homologous pair -- two genes may have the same allele (e.g. AA or aa) and individuals are said to be homozygous (again, "dominant" or "recessive" can be used) -- individuals with genes having two different alleles (e.g. Aa) are called heterozygous.
4. When an individual's allelic makeup is specified (e.g. AA, Aa or aa), this is called the individual's genotype -- the outward, physical expression of the genotype (e.g. purple flowered, white flowered) is called the phenotype.
5. When an individual produces gametes by meiosis, a gamete will only receive one (NOT two) allele.
Now, let's use these concepts to understand how Mendel obtained the results (e.g. 3:1 ratios):
1. P1 generation consisted of one parental line make up of homozygous dominant individuals (genotype AA) and another parental line of homozygous recessive individuals (genotype aa).
2. Each parental type produces a single kind of gamete: A-carrying gametes only from AA parents and only Ay-carrying gametes from aa parents.
3. F1 generation all heterozygous individuals (Aa) -- since A is dominant, these individuals have the purple flower phenotype.
Figure 10.6:
4. F2 obtained by selfing the F1 -- F1 individuals crossed with one another in what is called a monohybrid cross -- one trait; heterozygotes sometimes called "hybrids."
5. Each F1 parent (Aa) can produce two kinds of gametes (A-carrying gametes and a-carrying gametes) -- F2 produced by fertilization of 2 kinds of eggs (1/2 A and 1/2 a) by 2 kinds of pollen (1/2 A and 1/2 a).
6. Probability of A pollen fertilizing A egg = 1/2 X 1/2 = 1/4 (AA, purple)
Prob. of a pollen fertilizing A egg = 1/2 X 1/2 = 1/4 (Aa, purple)
Prob. of A pollen fertilizing a egg = 1/2 X 1/2 = 1/4 (Aa, purple)
Prob. of a pollen fertilizing a egg = 1/2 X 1/2 = 1/4 (aa, white)
Notice: the ratio of dominant to recessive phenotypes is 3:1 (3/4 : 1/4).
Figure 10.6:
Notice that phenotype for homozygous dominant and heterozygous individuals are the same (dominant phenotype) -- cannot tell genotype of dominant phenotype unless a test cross is performed.
Test cross -- crossing dominant phenotype (genotype unknown) with a homozygous recessive individual -- if dominant is homozygous, all offspring from cross will be dominant phenotype; if the individual is heterozygous, offspring will be 1/2 dominant and 1/2 recessive phenotype -- Mendel carried out test crosses on F1 individuals and got the predicted results -- this supported his hypothesis about segregation.
Test cross has practical applications -- used to determine whether individuals being considered for breeding stock carry undesirable recessive traits.
So, we've now covered the monohybrid and test crosses -- now, let's discuss the dihybrid cross -- experiments by Mendel in which he studied two separate traits simultaneously -- led him to formulate Mendel's Law of Independent Assortment = members of each pair of factors segregate independently of all other pairs -- now know that this is only true for genes that are on different chromosomes -- more on this later.
1.Mendel crossed true-breeding yellow (dominant), smooth-coated (dominant) seed plants (YYSS) with true-breeding green (recessive), wrinkled-coated (recessive) seed plants (yygg).
2. The F1 was all yellow, smooth plants (YySs).
3. F1 selfed (dihybrid cross) to get an F2.
4. Phenotypes of the F2 occurred in the following ratios:
a. yellow, smooth = 9/16
b. yellow, wrinkled = 3/16
c. green, smooth = 3/16
d. green, wrinkled = 1/16
5. More experiments with other pairs of traits also resulted in this characteristic 9:3:3:1 ratio.
Mendel noticed that the inheritance of each trait appeared to be independent of (unaffected by) the inheritance of the other trait.
1. Consider the yellow and green phenotypes alone -- according to Mendel's first law, a monohybrid cross will produce yellow to green in a 3/4 to 1/4 ratio.
2. Considering the smooth and wrinkled phenotypes alone, a monohybrid cross will produce 3/4 smooth to 1/4 wrinkled.
3. Law of probability states that the chance of two independent events occurring together is the product of their individual probabilities -- in the F2:
a. Prob. of being both yellow and smooth = 3/4 X 3/4 = 9/16.
b. Prob. of being both yellow and wrinkled = 3/4 X 1/4 = 3/16.
c. Prob. of being both green and smooth = 1/4 X 3/4 = 3/16.
d. Prob. of being both green and wrinkled = 1/4 X 1/4.
These results then are expected if the two traits being considered are in fact inherited independently of one another.
Note: Individuals with the dominant phenotype for two separate traits can have their genotype determined through a test cross with an individual that is homozygous recessive for the two traits:
a. offspring will all be double-dominant phenotype if unknown is homozygous dominant.
b. offspring will display 1:1:1:1 ratio if unknown is a double-heterozygote
I want to conclude this lecture by briefly mentioning several other pertinent points.
First, not all alleles for a particular trait display dominance-recessiveness. Many traits display various kinds of incomplete dominance -- one example of this is the phenomenon of codominance -- both alleles are expressed -- e.g. red bull x white cow = roan calf (both red and white hairs present).
Many genes have more than 2 alleles -- while an individual can only have 2 different alleles, a population can contain many different alleles.
Example: the ABO blood group is a single-gene trait -- there are 3 different alleles that determine a type of protein (antigen) found on the membrane of the red blood cell -- IO produces no antigen; IA produces type-A antigens; IB produces type-B antigens -- IA and IB are codominant with one another and both are dominant to IO.
Blood Groups Genotype RBC Antigen
Blood
Groups Genotype RBC
Antigen A IAIA,IOIO Type-A B IBIB,
IBIO Type-B AB IAIA Type-A and Type B O IOIO none
Blood group information is important in several ways.
1. blood transfusions -- donor and recipient blood types must be known to insure that transfusions are compatible.
2. paternity suits -- an analysis of blood types for a child, its mother and the child’s "alleged" father can be used to disprove, but not prove paternity.
e.g. child is type O, mother is type O, alleged father is AB -- this man cannot be father because an AB father and an O mother can only have children that are either type A or type B, but NOT type O.
Pleiotropy -- a single gene may have many phenotypic effects -- e.g. Sickle-cell disease is caused by an allele at the gene for hemoglobin -- besides effecting the shape of the red blood cell, this defect causes physical weakness, heart failure, impaired mental function, pneumonia, rheumatism and kidney failure.
Polygenic Inheritance -- many genes may interact with each other and with the environment to influence a single trait -- e.g. height, weight, intelligence.
Heritability -- extent to which variation in a traits controlled by genetic or environmental factors -- all variation due to genetic differences: h2 = 1.00; all variation due to environmental differences: h2 = 0.00 -- conception rate in cattle has a heritability of 0.05, while slaughter weight has a heritability of 0.85; human intelligence has heritability of 0.70-0.80.
Next time, we will discuss other aspects of genetics and inheritance.
NEXT TIME: Chromosomes and Genes.