for prof kankan

Biology 103 Laboratory Exercise – Genetic Problems Introduction Although the science of genetics has become a highly sophisticated discipline dealing with the interactions of hereditary factors at the molecular level, it has its roots in the basic laws of heredity initially discovered and presented by Gregor Mendel more than one hundred years ago. Mendel's success in discovering these laws was due largely to his application of the simple rules of mathematical probability - the laws of chance - to his observations concerning the inheritance of certain characteristics in the garden pea plant. Reginald Punnett and the Punnett Square The Punnett square is a diagram used by biologists to determine genotypic probability within the offspring from a particular genetic cross. The Punnett square shows every possible genotypic combination of maternal alleles with the paternal alleles for a genetic cross. Punnett squares only give probabilities for genotypes, not phenotypes. The square diagram was designed by the British geneticist, Reginald Punnett (1865-1967) and first presented to the science community in 1905. Punnett’s Mendelism (1905) is considered the first popular science book to introduce genetics to the public.

Solving Genetic Problems

R

R'

R

RR RR'

R'

RR' R'R'

Maternal alleles

A

A

a

Aa

Aa

Paternal Alleles

a

Aa

Aa

The first step in solving a genetic problem is to establish the genetic symbols you will use in your problem solution. Stay consistent by using these same symbols throughout the problem solving process. Represent dominant and recessive alleles (different forms of a gene) using traditional genetic symbols. Dominant alleles should be represented with the capital version of an alphabetic letter while using the lower case version to show recessiveness. For example: B = black color, b = white color.

Each individual gene or trait is diploid (2n) in nature and therefore, must be represented with two alleles. Continuing with the alleles mentioned previously, an individual may have the genetic makeup BB, Bb, or bb when using those alleles. Remember that gametes (sperm and egg) are haploid (n) and can only provide one allele per trait. For example: B or b An individual’s genotype contains the possible gametes that can be expected to be produced by that individual. Much of genetics revolves around the probability of the makeup of gametes. If the individual is homozygous, all of the gametes produced will possess the same kind of allele. For example, an individual with the genotype BB would be expected to produce only B gametes and individuals with genotype bb would produce only b gametes. If the individual is heterozygous, that is the individual’s genotype contains one dominant allele and one recessive allele (Bb), the gametes produced will possess one or the other of the two forms of the gene – B or b. When working a genetic problem, maintain the same order of alleles throughout the problem. This keeps the problem-solving process simple and consistent. Keep dominant alleles in front of recessive alleles. For example: Bb should not be written as bB. If you are working with more than one gene, remain trait specific. Never mix alleles from separate genes. Example: AaBb (correct), ABab (incorrect) Gene

Unit of inheritance

Allele

One of the alternating forms of a gene

Dominant

An allele that is expressed whether or not an alternate form of the allele is present; masks a recessive allele

Recessive

An allele that is expressed only when no alternate form of the allele is present

Homozygous

If the two alleles for a gene are either both dominant or both recessive, the individual is homozygous for that gene

Heterozygous

If the two alleles for a gene consist of a dominant allele and a recessive alele, the individual is heterozygous for that gene

Genotype

The allele makeup (AaBb) an organism possesses for a trait

Phenotype

The observable or physical portion of a trait an organism possesses; the appearance – tall, green, round, . . .

True breeding

Indicates an individual with a homozygous (dominant or recessive) genotype

Test cross

A genetic cross between a homozygous recessive individual and an individual for which you are trying to determine the genotype

F1

This symbol is used to designate the offspring from a given set of parents in a genetics problem; the first filial generation

F2

This symbol is used to designate the offspring resulting from inbreeding individuals of the F1; the second filial generation

Working genetic problems is one way of understanding how inheritance works. In this lab exercise, you will work several types of genetic problems. The genetic problems will increasingly become more complex. Be sure that you understand each type of problem before moving to the next. Monohybrid Monohybrid inheritance is the inheritance of a single characteristic. The different forms of the characteristic are usually controlled by different alleles of the same gene. For example, a monohybrid cross between two pure-breeding organisms (homozygous for their respective traits), one with tall height (dominant) and one with dwarf height (recessive), would be expected to produce an F1 generation with only tall height expressed in the offspring because the allele for tall height is dominant over the allele for dwarf height. Parental cross: Homozygous dominant (tall) X homozygous recessive (dwarf)

t

t

T

Tt Tt

T

Tt Tt

Dihybrid A dihybrid cross is often used to test for dominant and recessive genes in two separate characteristics. Such a cross has a variety of uses in Mendelian genetics. A dihybrid cross exemplifies Mendel’s Laws of Segregation and Independent Assortment and furthermore, the meiotic process of genetic recombination. For genes on separate chromosomes, each pair of alleles shows independent segregation. If the first filial generation (F1 generation) produces four offspring, the second filial generation, which occurs by crossing the members of the F1 generation, will have a phenotypic ratio of 9:3:3:1. The following example illustrates a dihybrid cross between two heterozygous pea plants. R represents the dominant allele for shape (round), while r represents the recessive allele (wrinkled). Y represents the dominant allele for color (yellow), while y represents the

recessive allele (green). Each plant has the genotype RrYy, and since the alleles for shape and color genes are independent, they can produce four types of gametes with all possible combinations: RY, Ry, rY and ry. Parental cross: heterozygote (shape and color) X heterozygote (shape and color)

RY Ry rY ry

RY RRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rY RrYY RrYy rrYY rrYy

ry RrYy Rryy rrYy rryy

In the offspring, the dominant alleles mask the recessive alleles producing nine genotypic combinations: nine combinations having round, yellow; three that are round, green; three that are wrinkled, yellow; and one that is wrinkled, green. The phenotypic ratio for this cross of 9:3:3:1 is typical for a dihybrid cross and is referred to as a Mendelian ratio. Incomplete dominance Incomplete dominance occurs the combined expression of two different alleles in the heterozygous condition produces a blending of the individual expressions of the two alleles. This heterozygous genotype creates an intermediate phenotype. Flower color in snapdragons is a classic example of incomplete dominance. For example, the heterozygous condition consisting of one allele for red flowers (R) and one allele for white (no pigment) flowers (R') is pink (R'R, or RR'). Both alleles are written with the same uppercase letter but with a prime or superscript number to differentiate the two. A classic example of incomplete dominance.

Parental cross: heterozygote (pink) X heterozygote (pink)

R

R'

R

RR RR'

R'

RR' R'R'

In the offspring genotypes, RR offspring make a lot of red pigment and appear red while R'R' offspring make no red pigment and appear white. Both RR' and R'R offspring make little pigment and appear pink. Codominance In codominance, neither phenotype is completely dominant. Instead, the heterozygous individual expresses both phenotypes. A common example is the ABO blood group system. The gene for blood types has three alleles: A, B, and O. O can also be written as the letter i. The A and B alleles are codominant to each other. The O allele is recessive to both A and B. So, an O allele in combination with anything other than another O allele will be masked and not expressed in the phenotype. A genotype with two O alleles produces type O blood. When a person possesses both an A allele and a B allele, they have type AB blood. Example Punnett square for a father with A and O, and a mother with B and O:

A

O

B

AB BO

O

AO OO

From the offspring genotypes, any of the blood types are possible for expression in an individual phenotype – AB, A, B, or O.

Monohybrid Cross 1. In dogs, the allele for long tail length (L) is dominant over the allele for short tail

length (l). If a dog heterozygous for tail length mates with a short-tailed dog, what will be the appearance of the F1? Determine the genotypic and phenotypic ratios. How many of the offspring will have long tails?

Work the Punnett Square.

Using your experience from Monohybrid Problem 1, work through the following monohybrid crosses. Configure the parental cross. Work the Punnett square. Determine the genotypic and phenotypic ratios. Answer the question within each problem.

2. In pea plants, the allele for tall stem length is dominant over the allele for dwarf length. If a pea plant homozygous for tall stem length is crossed with a dwarf plant, what will be the appearance of the F1? Determine the genotypic and phenotypic ratios. How many of the offspring will exhibit dwarf stem length?

3. Alberto has six fingers on each hand and six toes on each foot. His wife Alicia and their daughter Maria have the normal number of digits. Polydactyly (extra digits) is a dominant hereditary trait. What fraction of their children would be expected to have extra digits?

Dihybrid Cross 1. In guinea pigs, the alleles for black coat color (B) and short hair (S) are dominant

over the alleles for white coat color (b) and long hair (s). If a guinea pig homozygous dominant for both traits mates with a white-colored, long-haired guinea pig, what will be the appearance of the F1? Determine the genotypic and phenotypic ratios. How many of the offspring will be black in color?

Work the Punnett Square.

Using your experience from Dihybrid Problem 1, work through the following dihybrid crosses. Configure the parental cross. Work the Punnett square. Determine the genotypic and phenotypic ratios. Answer the question within the problem.

2. In watermelons, the alleles for green color and short shape are dominant over the alleles for striped color and long shape. A plant heterozygous for both traits is crossed with a plant with striped, long fruit. Determine the genotypic and phenotypic ratios. What percentage of the offspring will be striped color and short fruit?

3. In humans, the alleles for freckled pigmentation and dimpled cheeks are dominant over the alleles for non-freckled pigment and non-dimpled cheeks. Suppose a man and a woman with freckles and dimpled cheeks have two children: one child has freckles but no dimples and the other child has dimples but no freckles. Determine the genotypes of the parents.

Incomplete Dominance 1. In four o’ clocks, red flower color is incompletely dominant over white flower color

with the heterozygote exhibiting pink flower color. If a red flowered four o’clock is crossed with a pink flowered four o’clock, what will be the appearance of the F1 generation? Determine the genotypic and phenotypic ratios. How many of the offspring will have red flower color?

Work the Punnett Square.

Using your experience from the Incomplete Dominance Problem 1, work through the following incomplete dominance crosses. Configure the parental cross. Work the Punnett square. Determine the genotypic and phenotypic ratios. Answer the question within the problem.

2. In horses, pale colored coats are incompletely dominant over chestnut colored coats. When a pale colored horse and a chestnut colored horse mate all of the offspring are heterozygous for coat color producing a “palomino” colored coat. What will be the appearance of the F1 generation if two palomino colored horses mate? Determine the genotypic and phenotypic ratios. How many of the offspring have palomino coats?

3. In humans, the inheritance of curly hair illustrates incomplete dominance. When a

curly-haired individual reproduces with a straight-haired one, the children all have wavy hair. What offspring would be produced, in what proportions, when two people with wavy hair reproduce? When two people with curly hair reproduce? Determine the genotypic and phenotypic ratios for each cross.

Co-Dominance 1. On the same day in the same hospital ward, three baby girls are born at approximately

the same time to three different women. Information is lost in the nursery on delivery day and the hospital staff has to analyze the blood types of the babies and the parents to determine which baby belongs to whom.

From the following blood types, determine which baby belongs to which set of parents and the genotypes of all individuals. Determine the possible genotypes for each individual.

Work the Punnett Squares.

Brown 1 Brown 2

Greene 1 Greene 2

Greene 3 Greene 4

Jones 1 Jones 2

Which baby belongs to which set of parents?

Browns - Greenes - Joneses -

What are the actual genotypes for babies and parents?

Using your experience from Co-dominance Problem 1, work through the following co- dominance genetic cross. Configure the parental cross. Work the Punnett square. Determine the genotypic and phenotypic ratios. Answer the question within the problem.

2. A man with type O blood has a sister with type AB blood. What are the genotypes and phenotypes of their parents?

3. A woman names her former boyfriend in a paternity suit. Her child has blood type A.

The woman has blood type AB. The accused man has blood type B. Could this man be the father of her child?