How traits pass from parents to children
TEAS 7 outline: Apply concepts behind Mendel's laws of inheritance
Also covers HESI A2 Biology (Genetics)
- You inherit two alleles per gene, one from each parent.
- A dominant allele shows when present; a recessive one shows only with two copies.
- Yy × Yy gives a 3:1 phenotype and 1:2:1 genotype ratio.
- Incomplete dominance blends traits; codominance shows both, as in AB blood.
- X-linked recessive traits affect more males, who have only one X.
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Both parents are heterozygous: each has one dominant allele for yellow seeds, capital Y, and one recessive allele for green seeds, lowercase y. So both parents have yellow seeds.
Key facts
- Alleles are different versions of the same gene at the same location on homologous chromosomes; you get one from each parent, because each gamete carries only one allele per gene (law of segregation).
- Homozygous means two identical alleles (YY or yy); heterozygous means two different alleles (Yy).
- Genotype is the allele combination; phenotype is the observable trait. YY and Yy have the same dominant phenotype.
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- A cross of two heterozygotes (Yy × Yy) gives a 1:2:1 genotype ratio and a 3:1 dominant-to-recessive phenotype ratio.
- In incomplete dominance the heterozygote is intermediate (red × white snapdragons give pink); in codominance both alleles show at once (type AB blood).
- In ABO blood type, the Iᴬ and Iᴮ alleles are codominant and i is recessive, so type O is ii.
- Males have one X chromosome, so a single recessive X-linked allele causes the trait; fathers pass their X only to daughters, never to sons.
Quick check1 question · 2 more at the end
In pea plants, the allele for yellow seeds (Y) is dominant over the allele for green seeds (y). Which description fits a plant with the genotype Yy?
Correct answer: D.
A genotype with two different alleles, such as Yy, is heterozygous; two identical alleles (YY or yy) is homozygous. The phenotype is the trait you can see. Because Y is dominant, it masks y, so a Yy plant has yellow seeds, just like a YY plant. Green seeds appear only in yy plants.
A Yy has two different alleles, so it is not homozygous, and the dominant Y allele hides the green trait.
B The genotype term is right, but the recessive green phenotype shows only when no dominant Y allele is present.
C The phenotype is right, but homozygous means two identical alleles (YY), not Yy.
D Correct: Yy has two different alleles (heterozygous), and the dominant Y allele produces yellow seeds.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: You inherit two alleles for each gene, one from each parent. A dominant allele shows up whenever it is present; a recessive one shows only in a homozygous recessive individual. A Punnett square predicts offspring: two heterozygotes give a 3:1 phenotype ratio and 1:2:1 genotype ratio. Incomplete dominance blends traits, codominance shows both, and X-linked recessive traits affect more males.
1 What are genes and alleles?A gene is a section of DNA that affects a trait, such as seed color in pea plants. 1 min
A gene is a section of DNA that affects a trait, such as seed color in pea plants. Each gene can come in different versions called alleles. Your chromosomes come in pairs, one from each parent, so you carry two alleles for every gene on those pairs.
If both alleles are the same, you are homozygous for that gene. If they differ, you are heterozygous. A dominant allele (written with a capital letter, like Y) shows its effect whenever it is present. A recessive allele (lowercase, like y) shows only when there is no dominant allele to mask it.
2 What are Mendel's laws?The last two laws play out in meiosis I: homologous chromosomes separate (segregation), and each pair lines up in a random orientation (independent assortment). 1 min
Mendel's pea experiments led to three rules that every Punnett square relies on:
- Law of dominance: in a heterozygote, the dominant allele is expressed and hides the recessive one.
- Law of segregation: the two alleles of a gene separate when gametes form, so each egg or sperm carries just one. Each allele is equally likely to be passed on.
- Law of independent assortment: alleles of different genes sort into gametes independently, so inheriting Y says nothing about which seed-shape allele comes along. Genes linked close together on the same chromosome are the exception.
The last two laws play out in meiosis I: homologous chromosomes separate (segregation), and each pair lines up in a random orientation (independent assortment).
3 What is the difference between genotype and phenotype?Your genotype is the pair of alleles you carry. 1 min
Your genotype is the pair of alleles you carry. Your phenotype is the trait you can observe. In pea plants, yellow seed color (Y) is dominant over green (y):
| Genotype | Description | Phenotype |
|---|---|---|
| YY | Homozygous dominant | Yellow seeds |
| Yy | Heterozygous | Yellow seeds |
| yy | Homozygous recessive | Green seeds |
Notice that YY and Yy look identical. That is why you cannot read a genotype from a dominant phenotype. A test cross solves it: cross the yellow plant with a green (yy) plant. If every offspring is yellow, the parent was probably YY; if about half are green, it was Yy.
4 How do you use a Punnett square?A Punnett square lists the alleles one parent can pass on across the top and the other parent's down the side. 1 min
A Punnett square lists the alleles one parent can pass on across the top and the other parent's down the side. Each box is one equally likely offspring genotype. Here is a cross between two heterozygous plants (Yy × Yy):
| Y | y | |
|---|---|---|
| Y | YY | Yy |
| y | Yy | yy |
Read the four boxes:
- Genotype ratio: 1 YY : 2 Yy : 1 yy
- Phenotype ratio: 3 yellow : 1 green
- Each box is a 1 in 4 (25%) chance for every offspring.
This matches what Mendel saw. Crossing true-breeding violet-flowered and white-flowered pea plants gave all-violet offspring. Letting those offspring self-fertilize brought white flowers back in about a 3:1 ratio.
Two more crosses worth knowing by heart: Yy × yy gives 1 Yy : 1 yy (half dominant, half recessive), and YY × anything gives all dominant phenotype.
5 What if neither allele is fully dominant?In incomplete dominance, the heterozygote has an in-between phenotype. 1 min
In incomplete dominance, the heterozygote has an in-between phenotype. Crossing red-flowered and white-flowered snapdragons gives pink flowers. Cross two pink plants and you get 1 red : 2 pink : 1 white, so here the phenotype ratio matches the genotype ratio.
In codominance, both alleles are fully and separately expressed in the heterozygote. Human ABO blood type is the classic example. The Iᴬ and Iᴮ alleles are codominant, and i is recessive:
| Blood type | Possible genotypes |
|---|---|
| A | Iᴬ Iᴬ or Iᴬ i |
| B | Iᴮ Iᴮ or Iᴮ i |
| AB | Iᴬ Iᴮ |
| O | ii |
So a parent with type AB (Iᴬ Iᴮ) always passes either Iᴬ or Iᴮ, and a parent with type O always passes i.
6 How are sex-linked traits inherited?Females have two X chromosomes (XX); males have one X and one Y (XY). 1 min
Females have two X chromosomes (XX); males have one X and one Y (XY). Genes on the X chromosome but not the Y are X-linked. Because a male has only one X, a single recessive X-linked allele is enough to give him the trait. That is why X-linked recessive conditions such as hemophilia and color blindness show up more often in males.
A heterozygous female is a carrier: she usually shows no effects but can pass the allele on. Each son of a carrier mother has a 50% chance of inheriting it. A father passes his X only to his daughters and his Y to his sons, so a father cannot pass an X-linked trait to his son.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: A 3:1 ratio is a probability for each offspring, not a promise that exactly 1 of every 4 children will show the recessive trait.
Watch out: A dominant phenotype does not tell you the genotype; it could be homozygous or heterozygous, which a test cross can reveal.
Watch out: Incomplete dominance blends into a new middle phenotype; codominance shows both original phenotypes side by side, not a blend.
Watch out: Type A blood can be Iᴬ Iᴬ or Iᴬ i, not only Iᴬ Iᴬ.
Watch out: An affected father cannot pass an X-linked trait to his son; the son gets his Y chromosome.
How to remember it
Memory hook: "Geno is the code, pheno is the show." The genotype is the letters; the phenotype is what you see. For crosses, always write each parent's alleles first, then fill the square.
Self-check:
- Draw the Punnett square for Bb × bb and state the phenotype ratio.
- Explain why a type A parent and a type B parent can have a type O child.
- Say why a father with color blindness cannot pass it to his son.