Unit 8: Genetics and Heredity
Mendel's laws, Punnett squares, and how real human traits like blood type and color blindness get passed down.
9 topics · 28 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Mendel and the Birth of Genetics
- How it worksHow did Mendel's pea plant crosses reveal that traits are inherited as separate discrete units?
- Common mistakeWhy isn't a trait a blend of both parents' traits the way mixing two paint colors would be?
- Going furtherWhat happens to a recessive trait in a population if it never shows up for several generations?
Your turn1 of 2
Farmers and gardeners had bred plants for thousands of years before Mendel, yet none of them uncovered the mathematical rules of inheritance he found. Why did breeding thousands of pea plants let Mendel find hidden rules that centuries of casual observation missed?Genotype, Phenotype, and Alleles
- How it worksHow does having one dominant allele and one recessive allele still produce the dominant phenotype?
- Worked exampleFind the phenotype of an organism with genotype Bb where B for brown fur is dominant over b for white fur.
- Common mistakeWhy isn't the dominant allele automatically the more common one in a population?
Your turn1 of 2
Two brown-eyed people can carry noticeably different genotypes underneath their identical brown-eyed phenotype. Why can two people with brown eyes carry completely different combinations of eye color alleles?Punnett Squares and Monohybrid Crosses
- How it worksWalk me through filling in a Punnett square cell by cell to cross two heterozygous parents.
- Worked exampleFind the genotype ratios from crossing a Bb parent with a bb parent using a Punnett square.
- Worked exampleFind the probability of two Rr pea plants producing an rr offspring.
- Common mistakeWhy doesn't a Punnett square guarantee exactly a 3 to 1 ratio in every small batch of actual offspring?
Your turn1 of 2
A heterozygous Bb parent is crossed with a homozygous recessive bb parent. A Punnett square shows this cross produces offspring in a 1 to 1 ratio of Bb to bb. Out of 569 offspring from this cross, how many would be expected to show the dominant phenotype?Dihybrid Crosses and Independent Assortment
- How it worksHow does independent assortment let two different genes sort into gametes without affecting each other?
- Worked exampleFind the phenotype ratio from crossing two pea plants both heterozygous for seed shape and seed color.
- Common mistakeWhy doesn't a dihybrid cross always produce exactly the classic 9 to 3 to 3 to 1 ratio?
Your turn1 of 2
Crossing two pea plants heterozygous for both seed shape and seed color produces offspring in the classic 9 to 3 to 3 to 1 phenotype ratio. Out of 1134 offspring from this dihybrid cross, how many would be expected to show both dominant traits, the '9' portion of the ratio?Incomplete Dominance and Codominance
- How it worksHow does incomplete dominance blend two alleles into an intermediate phenotype?
- How it worksHow does codominance show both alleles fully instead of blending them, like in a roan horse's coat?
- Common mistakeWhy isn't incomplete dominance the same thing as codominance even though both look like a mix?
Your turn1 of 2
Crossing a red-flowered snapdragon with a white-flowered one sometimes produces offspring with entirely pink flowers, a color neither parent had. Why does this cross sometimes produce an all-pink offspring instead of red or white flowers?Multiple Alleles and Blood Type
- How it worksHow do the three alleles IA, IB, and i combine to produce the four human blood types?
- Worked exampleFind all possible blood types of children from a parent with genotype IAi and a parent with genotype IBi.
- Common mistakeWhy isn't type O blood the result of a completely missing gene rather than a specific allele?
Your turn1 of 2
A parent with genotype IAi is crossed with a parent with genotype IBi. This cross produces four equally likely offspring genotypes: IAIB, IAi, IBi, and ii, so one quarter of offspring are expected to have genotype ii, type O blood. Out of 569 children from this couple, how many would be expected to have type O blood?Sex-Linked Inheritance
- How it worksHow does a recessive allele on the X chromosome behave differently in males than in females?
- Worked exampleFind the probability a son inherits red-green color blindness from a carrier mother and an unaffected father.
- Common mistakeWhy can't a father pass an X-linked trait directly to his son?
Your turn1 of 2
A carrier mother (heterozygous for X-linked red-green color blindness) and an unaffected father have children together. Each son has a 1 in 2 chance of inheriting the colorblind X chromosome from his mother. Out of 285 sons born to this couple, how many would be expected to be colorblind?Reading a Pedigree
- How it worksHow do circles, squares, and shading in a pedigree encode who is affected by a trait?
- Worked exampleFind whether a trait is dominant or recessive in a pedigree where two unaffected parents have an affected child.
- Common mistakeWhy doesn't a trait skipping a generation in a pedigree mean it isn't genetic?
Your turn1 of 2
In a pedigree, two unaffected parents have one child who is affected by a genetic disorder, shown as a shaded shape. Why does this particular pattern reveal that the trait must be recessive rather than dominant?Polygenic Traits and Human Variation
- How it worksHow do several different genes each adding a small effect combine to produce continuous variation in a trait?
- Common mistakeWhy isn't height determined purely by genetics with no influence from nutrition or environment?
- Ties togetherHow does a polygenic trait like skin color differ from a single-gene trait like pea plant seed shape?
Your turn1 of 2
A trait controlled by one gene with a dominant and recessive allele, like pea plant seed shape, sorts people into a small number of distinct categories. Human height instead forms a smooth, continuous bell curve. Why does height form a smooth bell curve instead of just a few distinct categories?