Lecture 2.5 - IB 150

What is the probability that two heterozygous individuals (CFTR cftr) , will produce a child that is homozygous recessive w/ respect to this gene?

0.25

What is the probability that the heterozygous female will pass on a recessive allele of the CFTR gene?

0.5

What is the probability this child will suffer from cystic fibrosis?

0.5

Determining Potential Offspring

1. Determine all possible gametes of each parent (meiosis) 2. Determine all possible combinations of those gametes from two parents (fertilization) 3. For probability or proportions, divide number of possible geno-/phenotypes of interest by the total number of possible offspring 4. If given a total number of offspring and asked for expected numbers, multiply this fraction by the total number of offspring

What is the probability this individual is a carrier of a cystic-fibrosis causing allele?

1.0

What is the ratio of homozygous dominant to heterozygous to homozygous recessive genotypes?

1:2:1

What will be the phenotypic ratio of crossing two heterozygous individuals if this trait displays incomplete dominance?

1:2:1

What is the ratio of dominant to recessive phenotypes?

3:1

What is the probability that the resulting zygote inherited a recessive allele (cftr) from the mother?

50%

What would have to be the genotype of the second parent to result in a phenotypic ratio among their offspring of 1:0 dominant to recessive?

AA

What are the possible genotypes of an individual whose phenotype is "Blood Type A?'

AO or AA

What would have to be the genotypes of the parents to result in a phenotypic ratio among their offspring of 1:1 dominant to recessive?

Aa and aa

Both parents are heterozygous for the CFTR gene. What is their genotype?

CFTR cftr

Simple Genetic (Mendelian) Ratios

Four different combinations of maternal and paternal gametes that are equally likely

What does the genotype of an individual that displays the phenotype "Blood Type O" have to be?

OO

Why do you think the heterozygous individual is phenotypically healthy?

They have one allele that codes for a functional protein that can still perform the normal function

A few years this couple in fact has four offspring. How many of these real offspring are heterozygous?

cannot tell

The AND Rule

combined probability of two independent events equal the product of the probability of each independent event occurring

small gametes with a ....

flagellum (mobile tail)

syngamy

fusion of haploid gametes and ultimately of nuclei of gametes to produce a diploid zygote

gametes are produced from...

germline cells, stem cells that were set aside from the earliest stages of embryonic development to be the stem cells that will produce the primary oocytes and spermatocytes - the cells that will undergo meiosis to produce ova (egg cells) and seem cells

complete dominance

only phenotype of dominant allele is expressed because of presence of one functional allele is sufficient to provide needed function

large gametes

ova

codominance

phenotype exhibits BOTH phenotypic traits of either allele , as both allele yield functional (though different) proteins → Ex: human blood types (blood type A,B,AB)

incomplete dominance

phenotype is intermediate. this often results from an amount effect, where the one functional allele cannot produce enough of its gene product to yield the "wild-type" phenotype

gametogenesis is the...

production of gametes (sex cells)

If this couple had exactly four children, how many do you expect to be heterozygous?

two

Punnett square

visual representation of combined probability of two independent events

Basic Patterns of Inheritance

∙ Cystic fibrosis → ex of a single gene genetic disorder → disorder of the secretory glands → 1 in 4,000 births → common disease-causing allele: three base-pair deletion in CFTR gene → two copies of the disease-causing allele are required for the disease to display in the phenotype of the individual ∙ homozygous for "wild type" allele ∙ heterozygous ∙ homozygous for "cystic fibrosis" allele

The OR Rule of Calculating Probabilities

∙ If there is more than one way to achieve a combination of events and the order is not important, the probability of achieving all events with a particular combination is the sum of probabilities of each happening separately. ∙ What is the probability of the child being heterozygous? → The probability of a heterozygous offspring is.. - A recessive allele from mother and dominant from father OR a dominant from mother and recessive from father. Probability (heterozygous offspring) = 0.25 + 0.25 = 0.5

Which of the following is true?

∙ The A and B alleles are co-dominant with respect to each other ∙ The O allele is recessive with respect to both A and B alleles

blood types

∙ The ABO blood types are a result of a gene located on chromosome 9 ∙ In the case of the O allele, when compared to the A allele, lacks one nucleotide (guanine). This difference, which occurs at position 261, causes a frameshift that results in the premature termination of the translation and, thus, results in a loss of enzymatic activity. This results in the H antigen remaining unchanged in case of O groups.

gametogenesis

∙ male germ cell (2n) results in 4 sperm (n) (differentiation and maturation into gametes) → spermatogenesis ∙ female germ cell (2n) results in 1 ovum (egg) (n) → ovum has mass bulk of original cell → oogenesis (restricted to embryonic stage) ∙ the divisions are not equal

What can the genotypes of the gametes be that both individuals produce? Remember, both parents have the genotype: CFTR cftr

∙ two types of gametes: gamete 1- CFTR ; gamete 2- cftr