Unit 1: Mendelian Inheritance and Probability
Mendel's pea plants, the crosses that follow from his two laws, and the probability and chi-square tools that test whether real data fits them.
10 topics · 38 questions
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Mendel's Pea Plants and the Law of Segregation
- How it worksHow do two alleles for a gene separate into different gametes during the law of segregation?
- Worked exampleWalk me through predicting the offspring ratio when a heterozygous Tt pea plant self-fertilizes.
- Common mistakeWhy doesn't blending a tall allele and a short allele produce a medium-height plant?
- Going furtherWhat happens to the 3 to 1 ratio when the F2 generation is allowed to self-fertilize for another round?
Your turn1 of 2
A heterozygous Tt pea plant self-fertilizes and produces 151 offspring. Based on the 3 to 1 ratio, how many are expected to be short (tt)?Genotype, Phenotype, and the Monohybrid Cross
- How it worksHow does a Punnett square turn two parent genotypes into predicted offspring ratios?
- Worked exampleFind the genotype and phenotype ratios from crossing Bb x Bb for pea seed color.
- Common mistakeWhy isn't a heterozygote's phenotype automatically a mix of both alleles?
- Going furtherWhat happens to the offspring ratio when a heterozygote Bb is crossed with a homozygous recessive bb instead of another Bb?
Your turn1 of 2
A Bb x Bb cross for pea seed color produces 185 offspring. How many are expected to show the dominant phenotype?The Test Cross
- How it worksHow does crossing an unknown genotype with a homozygous recessive plant reveal what alleles it carries?
- Worked exampleFind the expected offspring ratio from a test cross of Bb x bb for flower color.
- Going furtherWhat happens to a test cross's results if the unknown parent turns out to be homozygous dominant?
Your turn1 of 2
A test cross of Bb x bb for flower color produces 124 offspring. How many are expected to show the recessive phenotype?Independent Assortment and the Dihybrid Cross
- How it worksHow do two genes on different chromosomes assort independently into gametes?
- Worked exampleFind the gamete types produced by a YyRr pea plant under independent assortment.
- Common mistakeWhy doesn't independent assortment mean two traits are always unrelated in every organism?
- Going furtherWhat happens to the dihybrid ratio when the two genes sit close together on the same chromosome instead of on separate ones?
Your turn1 of 2
A pea plant is heterozygous at 5 independently assorting genes. Under independent assortment, how many genetically distinct gamete types can it produce?The Dihybrid Punnett Square and the 9:3:3:1 Ratio
- How it worksHow does a 4 by 4 Punnett square generate the 9:3:3:1 phenotypic ratio?
- Worked exampleFind the phenotype ratio from crossing YyRr x YyRr for seed color and seed shape.
- Worked exampleFind how many of 320 offspring are expected to be yellow and wrinkled from a YyRr x YyRr cross.
- Common mistakeWhy doesn't a 9:3:3:1 ratio mean all four phenotype classes are equally likely to carry a recessive allele?
- Going furtherWhat happens to the phenotype ratio when one of the two genes shows incomplete dominance instead of full dominance?
Your turn1 of 2
A YyRr x YyRr cross for seed color and seed shape produces 494 offspring. Based on the 9:3:3:1 ratio, how many are expected to be yellow and wrinkled (the 3/16 recessive-shape class)?The Forked-Line Method and Trihybrid Ratios
- How it worksHow does the forked-line method break a trihybrid cross into three separate monohybrid ratios?
- Worked exampleFind the fraction of AaBbCc x AaBbCc offspring that are aabbcc using the forked-line method.
- Going furtherWhat happens to the forked-line method once a fourth independent gene gets added to the cross?
Your turn1 of 2
An AaBbCc x AaBbCc cross produces 1977 offspring. Using the forked-line method (1/4 x 1/4 x 1/4 for each recessive class), how many are expected to be aabbcc?The Product Rule and Sum Rule in Genetic Probability
- How it worksHow does multiplying independent probabilities give the chance of two traits occurring together?
- Worked exampleFind the probability that a child is a girl with attached earlobes given each trait has probability one half and one fourth.
- Worked exampleFind the probability of an Aa x Aa cross producing either an AA or an aa offspring using the sum rule.
- Common mistakeWhy isn't the probability of two independent traits ever added together instead of multiplied?
Your turn1 of 2
The probability a child is a girl is 0.539, and independently the probability a child has attached earlobes is 0.28. Using the product rule, what is the probability of a girl with attached earlobes?Conditional Probability in Genetic Crosses
- How it worksHow does updating on one child's known phenotype change the probability that a parent is a carrier?
- Worked exampleFind the probability that a woman is a carrier for an autosomal recessive disease given both her parents are carriers and she is unaffected.
- Common mistakeWhy doesn't having three unaffected children in a row make a fourth child any safer from a recessive disorder?
- Going furtherWhat happens to a carrier probability calculation once a direct genetic test result for one parent becomes available?
Your turn1 of 2
In a population, the carrier (Aa) frequency for an autosomal recessive disease is 0.224. Assuming random mating, what is the prior probability that two randomly chosen unaffected parents are both carriers AND their first child is affected? Multiply the chance both parents are carriers (f times f) by the one-in-four risk of an affected child from an Aa x Aa cross.The Chi-Square Test for a Genetic Cross
- How it worksHow does the chi-square statistic measure the gap between observed and expected offspring counts?
- Worked exampleFind the chi-square value for a cross giving 92 tall and 28 short plants against an expected 3 to 1 ratio out of 120.
- Common mistakeWhy doesn't a chi-square value of zero mean a cross proves Mendelian inheritance is true?
Your turn1 of 2
A cross expected to follow a 3 to 1 ratio produces 98 tall plants out of 146 total offspring (the rest are short). Compute the chi-square statistic: the sum over both categories of (observed minus expected) squared divided by expected, using expected counts of 0.75 times the total and 0.25 times the total.Degrees of Freedom and P-Values in Chi-Square Testing
- How it worksHow does the number of phenotype classes set the degrees of freedom in a chi-square test?
- Worked exampleFind the degrees of freedom and critical chi-square value at p equals 0.05 for a dihybrid 9:3:3:1 cross.
- Going furtherWhat happens to the conclusion of a chi-square test if the sample size is doubled but the ratios stay the same?
- Worked exampleFind the degrees of freedom for a trihybrid cross tested against a 27:9:9:9:3:3:3:1 ratio.
Your turn1 of 2
A cross is tested against a ratio with 6 phenotype classes. What are the degrees of freedom for its chi-square goodness-of-fit test?