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Gregor Mendel • The father of genetics • Grew pea plants, and observed the patterns of the baby pea plants he made and how they differed from their parents - heredity • http://www2.edc.org/weblabs/mendel/mendel.html
Mendel’s Conclusions • Mendel observed his pea plants varied in one characteristic in two ways (two varieties or traits) • Offspring of crossed plants appeared with one trait or the other for a given characteristic • Also observed… • Law of Dominance • Law of Segregation • Law of Independent Assortment
The Chromosomal Basis of Inheritance • Genetics are determined by what gametes your parents make during meiosis and which combine during reproduction • Probability
Chromosomal Basis of Inheritance • A single trait is located on a single chromosome • Each chromosome has a homologous pair, which means each trait appears on each homologous chromosomes, so there are 2 copies, or 2 alleles • The copies may be the same (homozygous) or different (heterozygous)
Chromosomal Basis of Inheritance and Crosses • For each characteristic, there is a dominant allele or form and a recessive allele. • The dominant allele is represented by an upper case letter, usually related to the trait • Ex. Green pod = G • The recessive allele is represented by the same letter but lower case • Ex. Yellow pod = g
Crosses • When determining heredity, each of the two alleles from the parent has an equal chance of combining with the allele from the other parent • Depending on which alleles combine determines what trait the offspring will have • Types of offspring created can be determined with a Punnett Square • Resultsof the cross are written as ratios, showing how many of a given phenotype or genotype will be present compared to the total number of options
Monohybrid Cross • The genetic makeup of an individual, or which chromosomes it has, is termed its genotype. • Aa, FF, jj • The physical traits that result from that are termed the phenotype. • Blond, tall, wrinkled
Monohybrid Cross • If an offspring receives two dominant alleles, it is homozygous dominant and displays the dominant trait • If an offspring receives two recessive alleles, it is homozygous recessive and displays the recessive trait • If an offspring receives one dominant and one recessive allele, the dominant allele masks the recessive allele, and the heterozygous offspring displays the dominant trait
For every genetic cross… • Make a complete Punnett square • Write the genotypic and phenotypic ratios • Genotypes should be written with the words homozygous and heterozygous
Examples • A purebred individual of a species is homozygous for a trait. In a certain flower, purple color is dominant to teal color. Perform a cross for a pure purple flower and another pure purple flower. Then perform a cross for a pure teal flower and another pure teal flower. Then perform a cross between a pure purple flower and a pure teal flower.
Examples • We are going to now consider a cross between two individuals that are heterozygotes for one trait: the shape of the pea in sweet pea plants which can be either round (R) or wrinkled (r). Round is completely dominant to wrinkled. We will conduct a monohybrid cross for pea shape. Our monohybrid cross example: Rr x Rr
Examples • In humans, brown eyes (B) are dominant over blue (b). A brown-eyed man marries a blue-eyed woman and they have three children, two of whom are brown-eyed and one of whom is blue-eyed. Draw the Punnett square that illustrates this marriage. What is the man’s genotype? What are the genotypes of the children?
Examples • In summer squash, white fruit color (W) is dominant over yellow fruit color (w). If a squash plant homozygous for white is crossed with a plant homozygous for yellow, what will the phenotypic and genotypic ratios be for: a. the F1 generation? b. the F2 generation? c. What will the phenotypic and genotypic ratios of the offspring be if you perform a testcross with the F1 generation?
Examples • In dogs, wire hair (S) is dominant to smooth (s). Cross ahomozygous wire-haired dog with a smooth-haired dog.
Examples • Woodrats are medium sized rodents with lots of interesting behaviors. You may know of them as packrats. Let's assume that the trait of bringing home shiny objects (H) is controlled by a single gene and is dominant to the trait of carrying home only dull objects (h). Suppose two heterozygous individuals are crossed. What are the genotypic and phenotypic ratios
Examples • The common grackle is a species of robin-sized blackbirds that are fairly common (hence the name) over most of the United States. Suppose that long tails (L) were dominant to short tails in these birds. A female short-tailed grackle mates with a male long-tailed grackle who had one parent with a long tail and one parent with a short tail. What is the male's genotype?
Examples • The ability to curl your tongue up on the sides (T, tongue rolling) is dominant to not being able to roll your tongue. A woman who can roll her tongue marries a man who cannot. Their first child has his father's phenotype. What are the genotypes of the mother, father, and child?
Examples • Saguaro cacti are very tall cylindrical plants that usually have two L-shaped arms, one on each side. Suppose you lived in southern Arizona where the Saguaro cactus is common and you happen to have one growing in your yard. Your Saguaro has two arms but one is longer than the other. Now, assume that arm length in these cacti are controlled by a single gene with arms of the same length (A) being dominant to arms of different lengths. What is the genotype of your cactus?
Examples • In dogs, there is an hereditary deafness caused by a recessive gene, “d.” A kennel owner has a male dog that she wants to use for breeding purposes if possible. The dog can hear, so the owner knows his genotype is either DD or Dd. If the dog’s genotype is Dd, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female (dd). Draw the Punnett squares to illustrate these two possible crosses. In each case, what percentage/how many of the offspring would be expected to be hearing? deaf? How could you tell the genotype of this male dog? Also, using Punnett square(s), show how two hearing dogs could produce deaf offspring.
Testcross • The previous cross is the definition of a testcross • Testing the genotype of a dominant phenotype by crossing it with a homozygous recessive individual • If recessive phenotype offspring are observed, then the unknown genotype must be heterozygote • Procedure…