Chapter 4 Section 2 Probability and Heredity

Probability is a number that describes how likely it is that an event will occur.

One way that probability can be expressed is through a percentage.

A Punnet square is a chart that shows all the possible combinations of alleles that can result from a genetic cross.

In a genetic cross, the allele that each parent will pass on to its offspring is based on probability.

Phenotypes and Genotypes

Phenotype is the outward expression of a gene or its physical traits.

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http://www.genome.gov/glossary.cfm?key=genetic%20marker

Genotype is an organisms genetic makeup or allele combination.

An organism that has two identical alleles for a trait is said to be homozygous.

An organism that has two different alleles for a trait is said to be heterozygous.

Codominance is when the alleles are neither dominant nor recessive, and both alleles are expressed independently and are uniquely recognizable in the offspring.

Let’s try some work on Punnet Squares.

    (Get it? "Square" = nerd. Ha ha ha ha ha ...)

Step # 1 is to draw your squares

Step #2 Write down your "cross" (mating).  Write the genotypes of the parents in the form of letters (ex: Tt x tt).

Step # 3"Split" the letters of the genotype for each parent & put them "outside" the p-square.

Step #4 - Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box. As

a general rule, the capital letter goes first and a lowercase letter follows.

Step # 5 List the possible genotypes and phenotypes of the offspring for this cross.

P-squARe prActICE QueSTioN #1 Let's say that in seals, the gene for the length of the whiskers has two alleles.  The dominant allele (W)

codes long whiskers & the recessive allele (w) codes for short whiskers.

a)  What percentage of offspring would be expected to have short whiskers from the cross of two long-whiskered seals, one that is homozygous dominant

and one that is heterozygous?

b) If one parent seal is pure long-whiskered and the other is short-whiskered, what percent of offspring

would have short whiskers?

P-sqARE PraCTice qUesTiON #2

In purple people eaters, one-horn is dominant and no horns is recessive.

Draw a Punnet Square showing the cross of a purple people eater that is hybrid for horns with a

purple people eater that does not have horns.

Summarize the genotypes & phenotypes of the possible offspring.

personally like to write down the info given in the question on my paper first.  So I start by writing: W = allele for long whiskers w = allele for short whiskers A homozygous dominant seal would be "WW" (homozygous dominant = 2 CAPITAL letters). A heterozygous seal would be "Ww" (heterozygous = 1 CAPITAL & 1 lowercase). The cross is in the question therefore: WW x Ww. The P-Square would look like this:

                                                       

The possible gametes from the homozygous parent seal are on the left in front of the rows, & the possible gametes from the heterozygous parent are above the columns. We fill in the boxes by copying "one letter from the left, one letter from the top". Analyzing our results, we find that 50% of our offspring (2 of 4 boxes) are "WW", and 50% (2 of 4 boxes) are "Ww". In terms of phenotype (what they would look like) 100% would have long whiskers (because all of the offspring have at least one "W", which codes for long whiskers). So the answer to question 1a is: 0% would have short whiskers.  The only way to have short whiskers is to be "ww", and that combo is not possible from the parents in this cross. b) If one parent seal is pure long-whiskered and the other is short-whiskered, what percent of offspring would have short whiskers? ANSWER: 0%.

P-sqARE PraCTice qUesTiON #2 - SOLUTION In purple people eaters, one-horn is dominant and no horns is recessive. Draw a Punnet Square showing the cross of a purple people eater that is hybrid for horns with a purple people eater that does not have horns. Summarize the genotypes & phenotypes of the possible offspring. ANSWER:

Genotypes of OffspringPhenotype(s) of

Offspring

50% hybrid (Hh) 50% homozygous

recessive (hh)

50% one-horn 50% no horns

No specific letter is given in the question to use as an abbreviation, so it's UP TO YOU!  Being a real rebel, I'll use this: H = dominant allele for one horn h = recessive allele for no (zero) horns A purple people eater that is "hybrid" has one of each letters (the definition of hybrid), so that parent is "Hh". A purple people eater without horns has the recessive phenotype and the only way to have a recessive phenotype is to have a homozygous recessive genotype, which is 2 lowercase letters, "hh". So our cross for this question is: Hh x hh. The p-square should be: