Bio 211 exam #4

(Mendel's Monohybrid Cross) Reveals principle of segregation

each diploid individual possesses two alleles at a locus - these two alleles separate when gametes are formed, one allele going into each gamete

Any discordance between monozygotic twins is usually due to...

environmental factors since monozygotic twins are genetically identical

Sex-linked characteristics are determined by...

genes located on sex chromosomes

Because identical twins share 100% of their genes and dizygotic twins share only ~50%,...

genetically influenced traits should exhibit higher concordance in monozygotic twins

When genes influence the trait, however, monozygotic twin pairs should exhibit...

higher concordance than dizygotic twin pairs because monozygotic twins share a greater percentage of their genes.

(X-Linked Recessive Traits) An XX person displaying an X-linked recessive trait must be...

homozygous --> all of their XY kids will display the trait

Monozygotic (identical) twins

result when a single egg, fertilized by a single sperm, splits early in development into two separate embryos (twins share 100% of genes)

High concordance in monozygotic twins by itself does not...

signal genetic influence since twins often in same environment

(Autosomal Recessive Traits) Most parents of affected offspring are heterozygous and unaffected; consequently, the trait seems to...

skip generations

(Autosomal Recessive Traits) When both parents are heterozygous...

~¼ offspring are expected to express the trait (will not be obvious unless the family is large)

(X-Linked Recessive Traits) When a XX person is heterozygous,...

~½ of XY children are affected and ½ of XX children will be unaffected carriers

Concordance

% of twin pairs that are concordant for a trait - If both members of a twin pair have a trait, the twins are concordant - If only one member of the pair has the trait, the twins are discordant

The binomial takes what form?

(p + q)^n - p = the probability of one event - q = the probability of the alternative event - n = the number of times the event occurs

Branch diagram

- Convenient way to organize all combinations of characteristics in the progeny of a cross - Can be used to predict genotypic or phenotypic ratios

Example, gene white in flies

- Encodes a transporter for pigments - When transporter protein is working: red color in eyes - When transporter protein is absent or nonfunctional: no pigment (white eyes) - In heterozygotes, there is still some transporter protein produced, and therefore eyes are still red

To evaluate the role of chance in producing deviations between observed and expected values, a statistical test called the chi-square goodness-of-fit test is used

- Gives information about how well observed values fit expected values - Indicates the likelihood that chance alone could produce the deviation between the expected and the observed values

Assigning genotypes to pedigrees- key skill

- Inspect the pedigree (use Table 6.1): - If trait is dominant, it should not skip generations nor be passed on to offspring unless parents have it - If trait is recessive, it may skip generations and will exist in carriers - Form a hypothesis for the mode of inheritance (e.g. autosomal dominant) - Deduce the genotypes in whole pedigree - Check that genotypes are consistent with phenotypes, especially in parents and kids - Revise hypothesis if necessary

Morgan studied inheritance of white eyes in Drosophila (fruit flies)

- Recall Mendel saw identical results when he did reciprocal crosses - Morgan's crosses demonstrated inheritance pattern for sex-linked genes - Since males possess only 1 X chromosome, they cannot be homozygous or heterozygous, and are said to be hemizygous

(Dihybrid Crosses) Helped establish the principle of independent assortment:

- States that alleles at different loci (different chromosomes) separate independently of one another - Is an extension of the principle of segregation that states that two alleles at a locus separate when gametes are formed - Segregation at each chromosome happens independently from other chromosomes

What is the purpose of a testcross and why is it useful?

- The purpose of a testcross is to reveal the genotype of the parent with an unknown genotype - This is useful since an individual with a dominant phenotype could have two possible genotypes, e.g. TT or Tt in tallness in plants

Autosomal dominant traits normally

- affect XX & XY individuals with equal frequency - do NOT skip generations

X-linked dominant traits normally...

- affect both XX and XY, although often appear more frequently in XX - do NOT skip generations

Y-linked traits normally...

- affect only XY people - are passed from XY parent to all XY kids - do NOT skip generations

At the end of your biology class, your professor asks you to develop a project to determine the genotype of a plant with red flowers. Red petal color (R) is dominant to pink flower color (r). To accomplish this task, you cross the plant with the unknown genotype with heterozygous red‑flowered plants. Which ratios are valid predictions of flower colors in the offspring? - all red flowers - all pink flowers - 3 red : 1 pink flowers - 1 red : 3 pink flowers

- all red flowers - 3 red : 1 pink flowers

Autosomal recessive traits normally...

- appear with equal frequency in XX & XY individuals - appear only when a person inherits two alleles for the trait, one from each parent

Genetic testing includes:

- prenatal testing (ultrasonography, amniocentesis, chorionic villus sampling, maternal blood screening) - preimplantation genetic diagnosis (used with in vitro fertilization) - screening for disease-causing alleles in newborns (e.g. 29 conditions) - detection of people heterozygous for recessive alleles (e.g Tay-Sachs) - pre-symptomatic testing for the presence of a disease-causing allele in at-risk people (e.g. autosomal dominant breast cancer and HD) - pharmacogenetic testing to evaluate the effectiveness and safety of drug treatment

When two organisms of known genotype are crossed, we expect certain ratios of genotypes and phenotypes in progeny

- these expected ratios are based on the Mendelian principles of segregation, independent assortment, and dominance - The ratios of genotypes and phenotypes actually observed among the progeny, however, may deviate from these expectations

(Mendel's Monohybrid Cross) the concept of dominance

--> When two different alleles are present in a genotype, only one allele may be expressed in the phenotype --> The dominant allele is the allele that is expressed, and the recessive allele is the allele that is not expressed

Suppose that two parents are both heterozygous for sickle cell anemia, which is an autosomal recessive disease. They have eight children. Use the binomial theorem to determine the probability that three of the children have sickle cell anemia and five of the children are healthy. Round your answer to the nearest tenth. Probability: ________

0.2

Modes of inheritance:

1. Autosomal recessive 2. Autosomal dominant 3. X-linked recessive 4. X-linked dominant 5. Y-linked 6. mitochondrial

(Branch diagram) If you are interested in predicting the genotype or phenotype of one specific outcome, you can calculate this by:

1. Break di- or tri-hybrid crosses into individual monohybrid crosses and use Punnett squares to predict individual probability ratios 2. For each trait, determine the probability of the individual outcome (using #1) then multiple individual probabilities to find the total (overall) probability

How to use branch diagram:

1. Break dihybrid cross into 2 monohybrid crosses and use Punnett squares to predict ratios 2. In 1st column, list proportions of phenotypes for 1 characteristic 3. In 2nd column, list proportions of phenotypes for next characteristic twice (next to each phenotype in 1st column) 4. Draw 'branches' to connect phenotypes, then multiply

How to use a Punnett square to predict the outcome of a genetic cross:

1. Draw a grid 2. List the gametes produced by one parent along the upper edge 3. List the gametes produced by the other parent down the left side 4. Fill each block within the Punnett square with an allele from each of the corresponding gametes (generates the genotype of the progeny produced by the fusion of those gametes in F1 progeny) 5. Write the phenotype expressed by each genotype; repeat for all the cells in the Punnett square Note difference between the genotypic and phenotypic ratios (what does each tell you?)

To use the chi-square test, you need to:

1.Determine the expected results (typically using a Punnett square) 2.Gather the observed results (will be given to you in problem) 3.Plug your numbers into the chi-square equation to get a X2 value 4.Determine the number of degrees of freedom (df) - df represent the # of ways the expected classes are free to vary - df = n -1; where n = the number of different expected phenotypes 5.Use a chi-square table to determine the probability associated with your calculated chi-square (X2) value 6. Probability is described in terms of the likelihood that you would reject your null hypothesis - Null hypothesis in this case is that there is no significant difference between observed and expected frequencies

Huntington's disease is an inherited autosomal dominant disorder that can affect both men and women. Imagine a couple has had seven children, and later in life, the husband develops Huntington's disease. He is tested and it is discovered he is heterozygous for the disease allele, Hh. The wife is also genetically tested for the Huntington's disease allele, and her test results show she is unaffected, hh. What is the percent probability that the first child of this couple will have Huntington's disease? probability: _______ What is the percent probability that four of the seven children will have Huntington's disease? probability: _______

50 27.34

(Dihybrid Crosses) Always obtained a ____________ ratio in F2

9:3:3:1

pedigree

A pedigree is a pictorial representation of a family history, essentially a family tree that outlines the inheritance of one or more characteristics - Note various shading patterns and what each means - Know how to interpret pedigrees, including all lines - symbols - "Female" = XX, "male" = XY

Many studies have suggested a strong genetic predisposition to migraine headaches, but the mode of inheritance is not clear. L. Russo and colleagues examined migraine headaches in several families, two of which are shown (L. Russo et al. 2005. Am J Hum Gen, 76:327-333). i. What is the most likely mode of inheritance for migraine headaches in these families? Explain your reasoning.

Autosomal dominant -The trait is most likely dominant because it does not skip generations, and affected individuals have one affected parent. -It is not X-linked because the affected XY individual II-8 has an unaffected XX child. For X-linked loci, an affected XY person would transmit the trait to all his XX offspring.

(The Binomial Expansion) What is it used to determine?

Essentially, it is used to determine the probability of a particular combination, rather than going through all possibilities

(Autosomal Dominant Traits) when do exceptions to the previous rule arise?

Exceptions to this rule arise when people acquire the trait as a result of a new mutation or when the trait has incomplete penetrance

A recessive mutant allele, black (b), causes a dark body color in Drosophila when homozygous. The normal wildtype color is described as tan. If a black female is crossed to a tan male whose father was black, what is the genotype of each parent? (The wild-type allele is B).

Genotype of female parent: bb (since she is black in color and black is recessive, bb is the only possibility) Genotype of male parent: Tan is either Bb or BB ...but recall his father was black (bb), --> so his father had to give him a "b" and his mother gave him "B" Therefore, male parent must be Bb

If the probability of blood-type A is ⅛ and the probability of being bloodtype O is ½, what is the probability of being either blood-type A or bloodtype O?

Since the question is asking about being either blood-type type A or bloodtype O, you should apply the addition rule: To add fractions, you must find common denominator...

In watermelons, bitter fruit (B) is dominant over sweet fruit (b), and yellow spots (S) are dominant over no spots (s). The genes for these two characteristics assort independently. A homozygous plant that has bitter fruit and yellow spots is crossed with a homozygous plant that has sweet fruit and no spots to produce F1 plants that are heterozygous BbSs. The F1 plants are intercrossed to produce the F2. A. What will be the phenotypic ratios in the F2?

The F1 are intercrossed to produce the F2: Bb Ss × Bb Ss. The F2 phenotypic ratios are as follows: - 9/16 bitter fruit and yellow spots (B_S_) - 3/16 bitter fruit and no spots (B_ss) - 3/16 sweet fruit and yellow spots (bbS_) - 1/16 sweet fruit and no spots (bbss)

binomial expansion

The binomial expansion can be used to calculate the probability of any specific set of outcomes among a large number of potential events

Method 2: Apply the Probability General Formula

The generalized formula is: Where n = total number of events p = probability of one event; q = probability of alternative event s = number of times outcome p occurs t = number of times outcome q occurs ! means factorial, so 4! = 4 x 3 x 2 x 1, etc. (Note 0! = 1)

Method 1: Apply the Expanded Binomial Equation

When one of two outcomes is possible during each of a succession of trials, (p + q)^n = 1 --> where p and q are probabilities of two possible outcomes and n = # of trials --> Recall algebraic method of expanding a binomial:

Genes on X chromosome determine ___________________; genes on Y chromosome determine __________________.

X-linked characteristics Y-linked characteristics

(X-Linked Dominant Traits) How to distinguish between X-linked and autosomal dominant:

XY: in X-linked dominant, XY person inherits trait only from XX parent (never XY), however in autosomal dominant, XY can inherit trait from XY XX: can receive an X-linked dominant trait from either parent - Affected XY pass the trait to all their XX and none of their XY kids (e.g. I-1) - Affected XX (if heterozygous) pass trait to ~ ½ their XY & ~ ½ their XX kids (e.g. as in children of III-6)

(Y-Linked Traits) Because each XY person has a single Y chromosome, there is only one copy of each Y-linked allele: therefore,...

Y-linked traits are neither dominant nor recessive - Recall from Ch 4, little genetic information is found on the human Y chromosome - Maleness (encoded by the SRY gene) is one of the few traits in humans that has been shown to be Y linked

A testcross is...

a cross between an individual with an unknown genotype and an individual with the homozygous recessive genotype

What will be the genotypic ratio in the offspring of two Aa parents that are crossed with each other? a. 1 : 2 : 1 b. 1 : 1 c. 3 : 1 d. 2 : 1 e. 9 : 3 : 3 : 1

a. 1 : 2 : 1

In humans, oculocutaneous (OCA) albinism is a collection of autosomal recessive disorders characterized by an absence of the pigment melanin in skin, hair, and eyes. That is, normal pigmentation (𝐴)(A) is dominant over albino characteristics (𝑎)(a) . For this question, assume the phenotype is determined by a single gene with two alleles. If both parents have normal pigmentation, what are all of the possible genotypes that may be observed in their offspring? a. AA, Aa, or aa b. Aa only c. aa only d. AA or Aa only

a. AA, Aa, or aa

The availability of direct-to-consumer genetic tests (e.g. 23 and Me) has raised concerns about the...

adequacy of the information provided and the absence of genetic counseling - Concerns have been raised about the privacy of genetic information and the potential for genetic discrimination - Some of those who do seek genetic testing pay for it themselves and use aliases to prevent the results from becoming part of their health records

(X-Linked Dominant Traits) Each person with X-linked dominant trait has...

affected parent

(Autosomal Recessive Traits) In the rare event that both parents are affected by an autosomal recessive trait...

all the offspring will be affected

(Mendel's Monohybrid Cross) Independent assortment

alleles at different loci separate independently

X-linked recessive traits normally...

appear more often in XY than XX individuals (need only 1 copy to display trait) - because XY get X from XX, often passed from unaffected XX --> affected XY --> unaffected XX (tends to skip generations) - are not passed from XY to XY (child inherits Y from parent in this case)

(X-Linked Recessive Traits) Obligate carriers

are are individuals whose heterozygous genotype can be definitively determined from a pedigree (e.g. XX individuals I-2, II-2, III-7)

Dizygotic (nonidentical) twins

arise when two separate eggs are fertilized by two different sperm, producing genetically distinct zygotes (twins share ~50% of genes, like any siblings)

(Autosomal Dominant Traits) Every person with a dominant trait must have inherited the allele from...

at least one parent (this is why autosomal dominant traits do not skip generations)

In pea plants, plant height is controlled by a single autosomal dominant gene. Tall plants (H) are dominant to short plants (h). In a cross of two tall heterozygous plants, which phenotype ratio is expected from the resulting offspring? a. 1:2:1 b. 1:1 c. 3:1 d. 9:3:3:1

c. 3:1

In pea plants, the allele for round seed shape, R, is completely dominant to the allele for wrinkled seed shape, r. Complete the Punnett square showing the genotypes possible among the offspring when two heterozygous individuals are crossed. Use the information from the Punnett square to answer the second question. In this cross between two heterozygous pea plants, what are the chances that an offspring with wrinkled seeds will be produced? a. 50% b. 75% c. 1% d. 25%

d. 25%

An individual possesses two alleles at a locus and these two alleles separate when gametes are formed, one allele going into each gamete. This genetic concept is known as the a. principle of independent assortment. b. concept of dominance. c. chromosome theory of heredity. d. principle of segregation. e. reciprocal cross.

d. principle of segregation.

Comparisons of adoptees with their adoptive parents and with their biological parents can therefore help to...

define the roles of genetic and environmental factors in the determination of human variation --> adoption studies were instrumental in showing that schizophrenia has a genetic basis and that obesity is at least partly influenced by genetics

Mendel's dihybrid crosses involved plants that...

differed in TWO characteristics

If high concordance is due to environmental factors, then...

dizygotic twins, who also share the same environment, should have same high concordance.

José is color blind, a somewhat rare trait. A pedigree for José's family is shown here. i. What is the most likely mode of inheritance for color blindness in José's family? ii. If José has a child with an XX individual who has no family history of color blindness, what is the probability that their first child will be XY and color-blind ?

i. What is the most likely mode of inheritance for color blindness in José's family? X-linked recessive. -The trait skips generations so it must be recessive. Only XY individuals have the trait, and they inherit the trait from their XX parents, who are carriers. It cannot be a Y-linked trait because none of the affected XY individuals inherited the trait from their XY parent. It is unlikely to be an autosomal recessive trait because we would not expect two unrelated XY folks marrying into the pedigree (José's XY parent and Patty's spouse) to be both carriers for a relatively rare trait. ii. José has a child with an XX individual who has no family history of color blindness, what is the probability that their first child will be XY and color-blind? - Barring a new mutation or nondisjunction, zero. José cannot pass his color-blind X chromosome to an XY child (José will pass a Y chromosome in this case).

Genetic counseling provides...

information and support to people concerned about hereditary conditions in their families.

A major extension of Mendelian principles is the pattern of...

inheritance of sex-linked characteristics

(Autosomal Dominant Traits) If an autosomal dominant allele is rare...

most people displaying the trait are heterozygous

Twins and adoptions provide...

natural experiments for separating the effects of genes and environmental factors on differences in traits.

The interpretation of genetic tests may be complicated by the presence of...

numerous causative mutations, incomplete penetrance, and the influence of environmental factors.

Recessive

protein product is not produced (lossof-function mutation)

Dominant

protein product is produced