10.1 Mendels Laws Gregor Mendel was a 19 th century Austrian monk who derived a series of laws that described the genetic patterns of heredity from one generation to the next. These laws account for the variability between members of a family. Mendel derived these laws from his experiments with pea plants.
10.1 Mendels Laws (cont.)
Mendels Experimental Procedure The study of peas was advantageous because they can both self-pollinate and cross-pollinate. –When peas self-pollinate, the progeny are genetically identical to the parent. –When peas cross-pollinate, variation occurs. Mendel tracked traits such as seed color, seed shape, and flower color.
Mendels Experimental Procedure (cont.) Mendel applied mathematical laws of probability to interpret the results. From the results Mendel derived the particulate theory of inheritance. The particulate theory of inheritance is based on the inheritance of particles from each parent, which we now know are genes.
Mendels Experimental Procedure (cont.)
One-Trait Inheritance Mendel performed crosses with true- breeding pea lines to observe the results. –The original parents were called the P generation. –The first generation of offspring were called the first filial (F 1 ) generation. –The second generation of offspring were called the second filial (F 2 ) generation.
One-Trait Inheritance (cont.) The results of genetic crosses can be predicted using a Punnett square. In a Punnett square, the possible male and female gametes of each parent are arranged on the horizontal and vertical axes. The squares represent every possible combination of gametes that could combine to form a zygote.
One-Trait Inheritance (cont.) The Punnett square can be used to understand the results of one of Mendels crosses. –When true-breeding tall plants were crossed with true-breeding short plants, the F 1 generation was all tall. –When the F 1 generation self-pollinated, ¾ of the progeny were tall and ¼ were short (3:1). –Thus the F 1 character for shortness was passed on by the tall F 1 generation.
One-Trait Inheritance (cont.) Mendels mathematical approach offered an explanation for the 3:1 pattern. –The F 1 parents each contained one copy of the hereditary factor for height, one dominant and one recessive (they are heterozygous for the factor). –The factors separated when the gametes were formed during meiosis, each gamete would get either the tall or short gene. –When random fusion of the gametes occurred during fertilization, the combinations were brought together in a 3:1 ratio, as indicated by the Punnett square.
One-Trait Inheritance (cont.)
One-Trait Testcross A testcross can be performed to determine if the F 1 progeny carries a recessive factor. A testcross crosses the F 1 generation with true-breeding tall plants to observe the distribution of height in the progeny. A 1:1 ratio of tall:short in the progeny confirms that a recessive factor is present.
One-Trait Testcross (cont.)
It is also possible that the F 1 generation carries two dominant factors (homozygous for the factor). If true, then the testcross would produce progeny that were all tall.
One-Trait Testcross (cont.)
From the results of these one-trait crosses, Mendel formed the law of segregation. –Each individual carries two factors for each trait. –The factors segregate (separate) when gametes form during meiosis. –Each gamete contains only one factor from each pair. –Fertilization gives each new individual two factors for each trait.
The Modern Genetics View Mendels results can be stated in more modern terms. –Traits are controlled by two alleles. –The dominant allele has the ability to mask the recessive allele. –The dominant allele is typically designated with a capital letter and the recessive a lowercase letter. –Alleles occur on homologous chromosomes at a specific location called the gene locus.
The Modern Genetics View (cont.)
Genotype Versus Phenotype It is possible that two organisms with different allelic combinations can have the same outward appearance. For example, if the dominant allele for finger length is S for short and the recessive allele is s for long, the individuals that are SS and Ss both have short fingers.
Genotype Versus Phenotype (cont.) Separate terms are needed to describe each condition. The genotype of an organism describes the two alleles that are present and the condition that this combination creates. For example, if an organism has two S alleles, the genotype is SS, or homozygous dominant for finger length.
Genotype Versus Phenotype (cont.) The phenotype of an organism refers to the physical appearance. For example, organisms that were either SS or Ss would have short fingers.
Genotype Versus Phenotype (cont.)
Two-Trait Inheritance Mendel also performed crosses between plants that differed in two traits. One example was a cross between tall plants with green pods and short plants with yellow pods. –Tall green pod plants are homozygous dominant for both traits (TTGG). –Short yellow pod plants are homozygous recessive for both traits (ttgg).
Two-Trait Inheritance (cont.) The progeny from the cross (the F 1 generation) were allowed to self-pollinate to generate the F 2 generation. From this cross, two possible patterns would be expected in the F 2 generation. –If the dominant factors (T and G) segregate during meiosis together, progeny will all be tall with green pods. –If the factors segregate separately, then four possible phenotype could result.
Two-Trait Inheritance (cont.) The results of the cross produced four phenotypes –Tall with green pods –Tall with yellow pods –Short with green pods –Short with yellow pods These results demonstrated that the two factors segregated independently.
Two-Trait Inheritance (cont.) Based upon these results, Mendel formulated the law of independent assortment. –Each pair of factors segregates (assorts) independently of other factors. –All possible combinations of factors occur in the gametes.
Two-Trait Inheritance (cont.)
Two-Trait Testcross Because the fruit fly Drosophila melanogaster has a variety of heritable mutations, this insect has been used extensively in genetic research. Two mutations displayed by this fly are wing length and body color. –Wild type (normal) flies have long wings and gray bodies. –Mutant flies can have short wings, ebony bodies, or both. –Both of these mutations are recessive traits.
Two-Trait Testcross (cont.) Fruit flies that have long wings and gray bodies could be heterozygous or homozygous for each trait. When the genotype is in doubt, it can be written as L__G__ to indicate that there is one dominant allele but the other is unknown. A two-trait testcross can be used to determine genotype of an L__G__ organism.
Two-Trait Testcross (cont.) In a two-trait testcross, a dominant L__G__ fly is crossed with a recessive fly of known genotype (llgg). A heterozygous dominant fly, or dihybrid, would produce four possible gametes. –LG –Lg –lG –lg
Two-Trait Testcross (cont.) The homozygous recessive fly can only form gametes containing lg. Since the homozygous parent only contributes lg, the other parent determines the phenotype of the offspring. Since the heterozygous parent would provide four gamete types, the offspring should be present in a 1:1:1:1 ratio.
Two-Trait Testcross (cont.)
But if the parent is homozygous dominant (LLGG), then the gametes would only contain LG. In this case, the testcross would produce offspring that had only the dominant phenotypes.
Mendels Laws and Probability Mendel realized that the results of his genetic crosses followed rules of probability. The rule of multiplication says that the chance of two events occurring together is the product of their chances of occurring separately. For example, the chance of getting two tails when you flip two coins is: ½ X ½ = ¼
Mendels Laws and Probability (cont.) Since alleles in a two-trait cross segregate independently, we can determine the probability of each allele pair. Ll X ll: Probability of ll = ½ Gg X gg: Probability of gg = ½ The probability of obtaining the llgg genotype: ½ X ½ = ¼
Mendels Laws and Meiosis Mendels laws are intimately related to the events of meiosis. The laws of segregation and independent assortment relate to the separation of homologous chromosomes during Meiosis I.
Mendels Laws and Meiosis (cont.)
10.2 Beyond Mendels Laws Mendels study of inheritance dealt with simple, independently-segregating traits. There are other patterns of inheritance other than the dominance/recessive relationship Mendel observed. The environment can also influence the phenotype of an organism.
Incomplete Dominance In incomplete dominance, the progeny show a phenotype intermediate to the parents. For example in four-oclock flowers, a cross between parents with red or white flowers yields progeny with pink flowers. In humans, wavy hair is an example of incomplete dominance for curly and straight hair.
Incomplete Dominance (cont.)
Multiple-Allele Traits For some factors, such as human blood type, there are more than two alleles. –I A = A antigen on red blood cells –I B = B antigen on red blood cells –i = Neither A or B antigen on red blood cells Each person has only two of these possible three alleles.
Multiple-Allele Traits (cont.) The combination of these alleles produce a persons blood type. –I A and I B alleles are dominant over i. –I A i and I A I A genotypes produce type A blood. –I B i and I B I B genotypes produce type B blood. –The ii genotype produces type O blood. –I A and I B alleles are codominant, meaning that neither is dominant over the other. –The I A I B genotype produces type AB blood.
Multiple-Allele Traits (cont.)
Polygenic Inheritance In polygenic inheritance, more than one pair of alleles determines the phenotype. Each dominant allele is additive to the overall phenotype. Human skin color is a polygenic trait. –Skin color is determined by three genes. –The more dominant alleles a person has, the darker their skin color.
Polygenic Inheritance (cont.)
Multifactorial traits are polygenes that are also controlled by environmental influences. There are several examples of multifactorial traits. –Skin color –Palate and lip disorders –Allergies –Some cancers
Environment and the Phenotype In some cases, the environment can affect phenotype more than genetics. For example, temperature can affect the color of primroses and Himalayan rabbits. The control of phenotype by genetics and/or environment leads to the nature versus nurture concept.
Environment and the Phenotype (cont.)
Pleiotropy Pleiotropy occurs when one gene has more than one effect. Pleiotropy is often seen in human disease, leading to syndromes, or groups of symptoms related to a genetic mutation. –Marfan syndrome –Sickle cell disease
10.3 Sex-Linked Inheritance The sex chromosomes, X and Y, not only determine gender but carry genes. The Y allele carries 26 genes mostly related to gender. The X allele carries genes for gender as well as genes unrelated to gender, the X- linked genes.
10.3 Sex-Linked Inheritance (cont.)
X-Linked Alleles Drosophila genetics can be used to understand sex linked genes. Consider a cross between a red-eyed female and a white-eyed male. –The F 1 generation all had red eyes. –The F 2 generation showed the typical 3:1 ratio, but all white-eyed flies were males. –The eye color gene is carried on the X chromosome as an X-linked, mutant allele.
X-Linked Alleles (cont.)
An X-Linked Problem X-linked genes are indicated as the X chromosome with an allele. –X R = red eyes in Drosophila –X r = white eyes in Drosophila There are several possible genotypes. – X R X R = red-eyed female –X R X r = red-eyed female –X r X r = white-eyed female –X R Y = red-eyed male –X r Y = white-eyed male
An X-Linked Problem (cont.) Heterozygous X R X r females are carriers of the white-eyed trait –The white-eyed trait is present but not shown. –Carriers are capable of passing that trait on to offspring. In this case, males cant be X-linked carriers because the X-linked gene has to be expressed.
10.4 Inheritance of Linked Genes Some genes are linked, meaning that they are carried on the same chromosome. Linked genes are inherited together because they cannot segregate during meiosis. The linked alleles on the same chromosome form a linkage group.
10.4 Inheritance of Linked Genes (cont.) Since Drosophila has only four chromosomes, there must be many genes on each. For example, chromosome II has genes for eye color, wing type, body color, leg length, and antennae type.
10.4 Inheritance of Linked Genes (cont.)
Constructing a Chromosome Map The relative position of genes on a chromosome can be illustrated with a chromosome map. The crossing-over that can occur during meiosis can be used to construct a chromosome map. Recall that if crossing-over does occur, the gametes produced are recombinant gametes.
Constructing a Chromosome Map (cont.)
Consider a cross between a heterozygous gray-bodied red-eyed fly and black-bodied purple-eyed fly. Because these traits are linked, a 1:1 ratio of the progeny is expected.
Constructing a Chromosome Map (cont.)
However, when the cross is performed a small percentage of the offspring have either gray bodies and purple eyes or black bodies and red eyes. This mixing of traits from the two parents occurs when there is crossing-over between the genes of a linkage group during meiosis I.
Constructing a Chromosome Map (cont.)
Linkage Data The frequency of a crossing-over event between two genes in a linkage group is proportional to the distance between them. By convention, a cross-over frequency of 1% indicates a distance between the two genes of 1 map unit. If the frequency of recombinant phenotypes in the offspring is 6%, the genes are 6 map units apart.
Linkage Data (cont.) The frequency of crossing-over can be used to map multiple genes on a chromosome. For example, assume a series of crosses were performed to determine the distance between three pairs of alleles on a single chromosome. –Black body –Purple eyes –Vestigial wings
Linkage Data (cont.) From the frequency of recombinant progeny, the distances between alleles are determined and a map created. –The distance between black-body and purple- eye alleles is 6 map units. –The distance between purple-eye and vestigial wing alleles is 12.5 map units. –The distance between black-body and vestigial wing alleles is 18.5 map units.