What did Mendel's pea experiments show about how traits pass from parents to offspring?

Traits are inherited as separate units, now called genes. A hidden factor can reappear in later generations, as the white trait did in about 1 in 4 F2 plants.

What does the law of segregation say, and why does it matter for gametes?

The two alleles for a gene separate during meiosis, so each gamete carries only one allele. This is why offspring get one allele from each parent.

What does the law of independent assortment say, and when does it fail?

Alleles of different genes sort into gametes independently of each other. It fails when genes are close together on the same chromosome, because those linked genes tend to be inherited together.

How do genotype and phenotype differ, and why can two organisms look the same but have different genotypes?

Genotype is the set of alleles an organism has, and phenotype is the trait you can observe. PP and Pp both show the dominant trait, so they look the same but have different genotypes.

Why might a small number of offspring not match a 3:1 ratio?

Punnett square ratios are probabilities, so each offspring has a chance of showing each phenotype. Real counts only approach the expected ratio when there are many offspring.

How does incomplete dominance differ from codominance?

In incomplete dominance, the heterozygote shows a blend of the two traits, such as pink from red and white parents. In codominance, both alleles show fully, such as red and white hairs together in roan cattle.

Why are X-linked recessive traits more common in males than in females?

Males have one X chromosome, so a single recessive allele on that X will show. Females need two copies of the recessive allele to show the trait, because a second X can carry the dominant allele.

How do you use a test cross to find out whether an unknown dominant organism is heterozygous or homozygous?

Cross the unknown with a homozygous recessive organism. If any offspring show the recessive trait, the unknown must be heterozygous; if all offspring show the dominant trait, it is most likely homozygous dominant.

How do you find the probability of a combined phenotype in a dihybrid cross without drawing the full square?

Find the probability for each gene separately and multiply them. For example, wrinkled and green from RrYy x RrYy is 1/4 x 1/4 = 1/16.

What pedigree pattern suggests that a trait is recessive?

Two unaffected parents have an affected child. The affected child must have received a recessive allele from each parent, so both parents are carriers if the trait is autosomal.

Biology

Mendelian genetics: flashcards, quiz and summary

A free study pack Chomp made from a page of notes on this topic: 10 flashcards, a 7-question quiz, the test traps and a summary. No account needed.

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The short version

Mendelian genetics explains how traits pass from parents to offspring as separate units, now called genes. Gregor Mendel found the basic rules in the 1860s by breeding pea plants and counting the offspring, and Punnett squares let you predict the ratios for one-gene, two-gene, and sex-linked crosses.

Flashcards

1What did Mendel's pea experiments show about how traits pass from parents to offspring?
Traits are inherited as separate units, now called genes. A hidden factor can reappear in later generations, as the white trait did in about 1 in 4 F2 plants.
2What does the law of segregation say, and why does it matter for gametes?
The two alleles for a gene separate during meiosis, so each gamete carries only one allele. This is why offspring get one allele from each parent.
3What does the law of independent assortment say, and when does it fail?
Alleles of different genes sort into gametes independently of each other. It fails when genes are close together on the same chromosome, because those linked genes tend to be inherited together.
4How do genotype and phenotype differ, and why can two organisms look the same but have different genotypes?
Genotype is the set of alleles an organism has, and phenotype is the trait you can observe. PP and Pp both show the dominant trait, so they look the same but have different genotypes.
5Why might a small number of offspring not match a 3:1 ratio?
Punnett square ratios are probabilities, so each offspring has a chance of showing each phenotype. Real counts only approach the expected ratio when there are many offspring.
6How does incomplete dominance differ from codominance?
In incomplete dominance, the heterozygote shows a blend of the two traits, such as pink from red and white parents. In codominance, both alleles show fully, such as red and white hairs together in roan cattle.
7Why are X-linked recessive traits more common in males than in females?
Males have one X chromosome, so a single recessive allele on that X will show. Females need two copies of the recessive allele to show the trait, because a second X can carry the dominant allele.
8How do you use a test cross to find out whether an unknown dominant organism is heterozygous or homozygous?
Cross the unknown with a homozygous recessive organism. If any offspring show the recessive trait, the unknown must be heterozygous; if all offspring show the dominant trait, it is most likely homozygous dominant.
9How do you find the probability of a combined phenotype in a dihybrid cross without drawing the full square?
Find the probability for each gene separately and multiply them. For example, wrinkled and green from RrYy x RrYy is 1/4 x 1/4 = 1/16.
10What pedigree pattern suggests that a trait is recessive?
Two unaffected parents have an affected child. The affected child must have received a recessive allele from each parent, so both parents are carriers if the trait is autosomal.

Practice quiz

Pick an answer to check it.

1In a cross of Pp x Pp, what fraction of offspring should show the recessive white phenotype?
Show the answer
D. 1 in 4 Pp x Pp gives 1 PP : 2 Pp : 1 pp. Only the pp offspring show the recessive trait, which is 1 out of 4 boxes.
2Red snapdragons (RR) are crossed with white snapdragons (WW), and the resulting pink offspring (RW) are crossed with each other. What phenotype ratio should appear in the next generation?
Show the answer
B. 1 red : 2 pink : 1 white This is incomplete dominance. RW x RW gives 1 RR (red), 2 RW (pink), and 1 WW (white), so the phenotype ratio is 1 red : 2 pink : 1 white.
3A carrier mother (X^H X^h) and an unaffected father (X^H Y) have a son. What is the chance that the son is affected by an X-linked recessive trait?
Show the answer
C. 50% The mother gives an X^h to half of her sons, and a son who receives X^h from his mother and Y from his father is affected. Each son has a 50% chance.
4In a dihybrid cross of RrYy x RrYy, what fraction of offspring should be wrinkled and green?
Show the answer
C. 1/16 Wrinkled is rr (1/4) and green is yy (1/4). Multiplying them gives 1/4 x 1/4 = 1/16, which matches the one wrinkled green box in the 16-box square.
5In a pedigree, two unaffected parents have an affected child. What is the most likely conclusion?
Show the answer
D. The trait is recessive Two unaffected parents with an affected child means the child received a recessive allele from each parent. The trait is recessive, and if it is autosomal both parents are carriers.
6Which genotype is possible for a person with type A blood in the ABO system?
Show the answer
B. IAIA Type A blood is IAIA or IAi. IAIB is type AB, IBIB is type B, and ii is type O, so only IAIA fits type A here.
7An unknown purple pea plant is crossed with a white pea plant, and all the offspring are purple. What can you conclude about the unknown plant?
Show the answer
A. It is homozygous dominant (PP) If the unknown were Pp, about half the offspring would be white. Since all offspring are purple, the unknown is most likely PP, because PP x pp gives all purple Pp offspring.

Test traps

What teachers like to test on this, and the mistakes students make.

Confusing dominant with common: a dominant allele is not more frequent in a population, and Huntington's disease is rare even though it comes from a dominant allele.
Mixing up genotype and phenotype, such as saying a purple plant is PP without a test cross, since PP and Pp both look purple.
Treating a Punnett square ratio as a guarantee, or expecting small families to match 3:1 exactly.
Confusing incomplete dominance (a blend, such as pink) with codominance (both traits fully shown, such as red and white hairs).
Forgetting that the dihybrid 9:3:3:1 ratio assumes independent assortment, and missing that linked genes change the expected ratio.
Getting the sex-linked inheritance wrong, such as saying a father can pass an X-linked allele to his son, or forgetting that the Y chromosome carries few genes.

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Summary

Traits are passed from parents to offspring as separate units called genes, and Mendel's pea experiments showed the basic rules.
In Mendel's cross of purple and white peas, all F1 plants were purple, but about 1 in 4 F2 plants was white, so the white factor was hidden rather than lost.
The law of segregation says the two alleles for a gene separate during meiosis, so each gamete carries only one allele.
The law of independent assortment says alleles of different genes sort into gametes independently, unless the genes are linked on the same chromosome.
A Punnett square shows the probability of each genotype and phenotype, but each box is a chance, not a guarantee.
Incomplete dominance blends the two alleles in the heterozygote, while codominance shows both alleles fully.
Sex-linked recessive traits such as red-green color blindness and hemophilia appear more often in males because males have only one X chromosome.
Pedigrees let you infer whether a trait is dominant or recessive and whether it is autosomal or X-linked.

Key terms

Gene: A segment of DNA that codes for a trait.
Allele: A version of a gene, such as P or p for flower color in peas.
Dominant allele: An allele, written with a capital letter, that shows whenever it is present.
Recessive allele: An allele, written with a lowercase letter, that shows only when an organism has two copies of it.
Genotype: The alleles an organism has for a gene, such as PP, Pp, or pp.
Phenotype: The observable trait an organism shows, such as purple flowers.
Homozygous: Having two identical alleles for a gene, such as PP or pp.
Heterozygous: Having two different alleles for a gene, such as Pp.
Incomplete dominance: A pattern where the heterozygote shows a blend of the two homozygous traits, such as pink snapdragons from red and white parents.
Codominance: A pattern where both alleles show fully in the heterozygote, such as the red and white hairs of roan cattle.
Test cross: A cross between an organism with the dominant phenotype and a homozygous recessive organism, used to find whether the unknown is homozygous or heterozygous.
Linked genes: Genes located close together on the same chromosome that tend to be inherited together, which is an exception to independent assortment.

How to study it

Monohybrid crosses and Punnett squares

Write the parent gametes along the top and side of the square, then fill each box with one allele from each parent.
For Pp x Pp, the genotype ratio is 1 PP : 2 Pp : 1 pp and the phenotype ratio is 3 purple : 1 white.
Use a test cross to find an unknown dominant-looking organism: any recessive offspring means the unknown is heterozygous.
Remember that each box is a 25% chance for each offspring, so small families may not match the ratio.

Dihybrid crosses and independent assortment

For RrYy x RrYy, each parent makes four gamete types (RY, Ry, rY, ry), so the square has 16 boxes.
The phenotype ratio is 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
To find the probability of a combined phenotype quickly, multiply the probabilities for each gene, such as 1/4 x 1/4 = 1/16 for wrinkled green.
Linked genes on the same chromosome do not follow the 9:3:3:1 ratio because they tend to be inherited together.

Beyond simple dominance and sex-linked traits

In incomplete dominance, the heterozygote is in between the parents, so Rr-type crosses give blended phenotypes.
In codominance, the heterozygote shows both traits fully, as with ABO blood type where IA and IB are codominant.
Human females are XX and males are XY, so males have only one X and show any X-linked recessive allele they carry.
A father passes his X to every daughter and his Y to every son, so a father cannot pass an X-linked trait to a son.

Reading a pedigree

Squares represent males, circles represent females, and shaded shapes represent affected individuals.
If two unaffected parents have an affected child, the trait is most likely recessive, and if it is autosomal both parents are carriers.
If every affected person has an affected parent, the trait is likely dominant.
If mostly males are affected and the trait passes through unaffected mothers, the trait is likely X-linked recessive.

The notes this pack came from

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Big idea

Traits pass from parents to offspring as separate units, now called genes. Gregor Mendel found the basic rules in the 1860s by breeding pea plants and counting offspring.

Mendel's experiments

Peas grow fast, have traits with two clear forms (purple or white flowers) and allow controlled pollination.
He crossed true-breeding parents (P generation) to get the F1, then self-pollinated the F1 to get the F2.
Purple × white gave all purple F1 plants. In the F2, about 1 in 4 plants was white (3:1), so the white "factor" was hidden, not lost.

Key vocabulary

Gene: a DNA segment that codes for a trait. Allele: a version of a gene (P or p).
Dominant allele (capital letter): shows whenever present. Recessive allele (lowercase): shows only with two copies.
Genotype: the alleles an organism has (PP, Pp, pp). Phenotype: the observable trait (purple flowers).
Homozygous: two identical alleles (PP or pp). Heterozygous: two different ones (Pp).

Mendel's two laws

Law of segregation: the two alleles for a gene separate during meiosis, so each gamete carries only one.
Law of independent assortment: alleles of different genes sort into gametes independently. Exception: genes close together on one chromosome are linked and tend to be inherited together.

Monohybrid Punnett squares

Parent gametes go on the sides; fill in the boxes.
Pp × Pp: genotypes 1 PP : 2 Pp : 1 pp (1:2:1); phenotypes 3 purple : 1 white (3:1).
Each box is a 25% chance per offspring, not a guarantee.
Test cross: cross a dominant-looking organism with a homozygous recessive one. Any recessive offspring means the unknown was heterozygous (Pp × pp gives 1:1).

Dihybrid cross

Two genes: seed shape (R round, r wrinkled) and seed color (Y yellow, y green).
RrYy × RrYy: each parent makes 4 gamete types (RY, Ry, rY, ry), so the square has 16 boxes.
Phenotypes: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
Shortcut: multiply probabilities. Wrinkled and green = 1/4 × 1/4 = 1/16.

Beyond simple dominance

Incomplete dominance: the heterozygote is in between. Red (RR) × white (WW) snapdragons give all pink (RW). Two pinks give 1 red : 2 pink : 1 white.
Codominance: both alleles show fully. Roan cattle have both red and white hairs.
ABO blood type has three alleles: IA and IB are codominant, i is recessive. Type A: IAIA or IAi. Type B: IBIB or IBi. Type AB: IAIB. Type O: ii.

Sex-linked traits

Human females are XX, males XY. The Y carries few genes.
Males have one X, so one recessive X-linked allele shows up. So red-green color blindness and hemophilia are more common in males.
Carrier mother (X^H X^h) × unaffected father (X^H Y): each son has a 50% chance of being affected; daughters are not affected, but each has a 50% chance of being a carrier.
A father passes his X to every daughter and never to a son.

Reading a pedigree

Squares are males, circles females, shaded shapes affected. A horizontal line joins parents; a vertical line leads to children.
Two unaffected parents with an affected child: the trait is recessive. If it is autosomal, both parents are carriers.
Every affected person has an affected parent: likely dominant.
Mostly males affected, passed through unaffected mothers: likely X-linked recessive.

Common mix-ups

Dominant does not mean common. Huntington's disease comes from a dominant allele but is rare.
Same phenotype does not mean same genotype (PP and Pp both look purple).
Ratios are probabilities. Real counts only approach 3:1 with many offspring.
Incomplete dominance blends (pink); codominance shows both (red and white hairs).

Questions

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