Heredity · Lesson 4 of 7
Rules for the Inheritance of Traits – Mendel’s Contributions
“Mendel counted peas so carefully that genetics finally revealed its pattern.”
• Explain why offspring receive two versions of a gene. • Reconstruct Mendel’s monohybrid cross and its ratios. • Distinguish dominant, recessive, genotype and phenotype. • Construct and interpret inheritance grids. • Explain independent inheritance and new trait combinations. • Reason backwards from offspring to possible parental genotypes.
Gregor Mendel turned inheritance into a quantitative science. Instead of merely noticing that offspring resembled their parents, he selected clear contrasting characters in pea plants, controlled the crosses, counted large numbers of offspring and compared generations. The numerical patterns revealed rules that could not be seen from a single plant.
Mendel’s Contributions
Garden peas offered contrasting visible characters such as tall or short stems, round or wrinkled seeds, violet or white flowers, and yellow or green seeds. Peas could self-pollinate to maintain a form or be cross-pollinated to combine different forms. Mendel’s crucial advance was to count individuals showing each form in successive generations.
In sexual reproduction, the father and mother contribute practically equal amounts of genetic material. An offspring therefore receives two copies of a gene for a trait, one from each parent. The two copies may be identical or different. A letter model helps track them: capital T can represent the form associated with tallness and lowercase t the form associated with shortness.
A gene is a section of DNA that carries information for a functional product and influences a characteristic.
An allele is an alternative form of a gene, such as T or t in a simplified height model.
The genotype is the combination of alleles carried by an organism for a trait, such as TT, Tt or tt.
The phenotype is the observable expression of a characteristic, such as tall or short.
A homozygous genotype contains two identical alleles for a gene, such as TT or tt.
A heterozygous genotype contains two different alleles for a gene, such as Tt.
A heterozygous organism carries the recessive allele even when the dominant phenotype is visible. This distinction is essential when reasoning backwards from offspring to a parent’s possible genotype.
Monohybrid Inheritance
Mendel crossed pure tall plants with pure short plants. All first-generation progeny were tall; there were no medium-height plants. The short form had not disappeared from the hereditary information, because self-pollinating the first-generation tall plants produced short plants again in the second generation. About three quarters were tall and one quarter short.
A dominant trait is expressed when at least one copy of its corresponding dominant allele is present in the simplified Mendelian model.
A recessive trait is expressed only when both gene copies are the recessive form in the simplified Mendelian model.
The first-generation tall plants were Tt: they carried both T and t, but T was expressed. During germ-cell formation, the two alleles separated, so half the gametes carried T and half carried t. Random fertilisation produced TT, Tt, Tt and tt combinations. This gives a genotype ratio of 1 : 2 : 1 and a phenotype ratio of 3 tall : 1 short.
How To Solve A Monohybrid Cross
| Step | Action | Check |
|---|---|---|
| Identify the forms | Choose symbols and state which is dominant | Use the same letter for both alleles |
| Write parental genotypes | Translate the given phenotypes and ancestry | A dominant phenotype may be TT or Tt |
| List gametes | Put one allele from each parent into each gamete | A gamete carries one allele |
| Complete the grid | Combine one maternal and one paternal allele in every cell | Every offspring has two alleles |
| Count outcomes | Separate genotype and phenotype counts | Do not confuse 1 : 2 : 1 with 3 : 1 |
Problem
Predict the first-generation offspring of TT × tt.
- 1.Given: pure tall TT and pure short tt.
- 2.TT produces only T gametes; tt produces only t gametes.
- 3.Every fertilisation combines T with t.
- 4.All offspring are Tt.
- 5.Because T is dominant, all offspring are tall.
Problem
Find the genotype and phenotype ratios for Tt × Tt.
- 1.Each parent produces T and t gametes.
- 2.Complete four combinations: TT, Tt, Tt and tt.
- 3.Genotypes: one TT, two Tt, one tt, giving 1 : 2 : 1.
- 4.Phenotypes: TT and Tt are tall; tt is short.
- 5.The phenotype ratio is 3 tall : 1 short.
Problem
A tall plant crossed with a short tt plant produces a short offspring. What is the tall parent’s genotype?
- 1.A short offspring must be tt.
- 2.The short parent supplies one t.
- 3.The tall parent must also have supplied t.
- 4.A tall plant carrying t cannot be TT; it must be Tt.
- 5.Conclusion: the tall parent is heterozygous Tt.
Confirming The Combination Ratio
Visible tall plants in the second generation include both TT and Tt, so appearance alone cannot separate them. To confirm the 1 : 2 : 1 genotype ratio, each tall plant can be crossed with a short tt plant. A TT plant produces only tall offspring, whereas a Tt plant can produce tall and short offspring. The pattern reveals which tall plants carry the hidden recessive allele.
Independent Inheritance
Mendel also crossed plants differing in two characteristics. For seed shape and colour, the first-generation plants showed the dominant forms. Self-pollination produced parental combinations and new combinations, such as round green and wrinkled yellow. The appearance of these new combinations showed that the factors controlling the two traits could be inherited independently.
For a simplified RrYy × RrYy cross, each parent can form four gamete types: RY, Ry, rY and ry. A four-by-four grid gives sixteen equally represented combinations. Grouping by phenotype produces 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
How To Solve A Dihybrid Cross
Treat each gene pair separately when listing gametes. A parent RrYy contributes one allele for seed shape and one for seed colour to each gamete, producing RY, Ry, rY and ry. Combine each gamete from one parent with every gamete from the other, then group the sixteen genotypes by visible phenotype. Finally check that all phenotype counts add to sixteen.
Problem
List the gametes produced by RrYy.
- 1.Choose one allele from the R/r pair and one from the Y/y pair.
- 2.R can combine with Y or y, producing RY and Ry.
- 3.r can combine with Y or y, producing rY and ry.
- 4.The four gamete types are RY, Ry, rY and ry.
Problem
Why can RrYy parents produce a wrinkled yellow offspring?
- 1.Wrinkled requires rr, so each parent must contribute r.
- 2.Yellow requires at least one Y, so the offspring may be rrYY or rrYy.
- 3.Gametes carrying rY or ry can combine to create these genotypes.
- 4.Wrinkled yellow is a new combination if the original parents showed round yellow and wrinkled green.
- 5.Its appearance supports independent inheritance.
Problem
In an RrYy × RrYy cross, find the probability of wrinkled green offspring.
- 1.For seed shape, rr occurs with probability 1/4.
- 2.For seed colour, yy occurs with probability 1/4.
- 3.The traits assort independently in this model, so multiply: 1/4 × 1/4 = 1/16.
- 4.Thus one of sixteen expected offspring is wrinkled green.
- 5.This agrees with the 9 : 3 : 3 : 1 grouping.
Dominant does not mean stronger, better or more common. A 3 : 1 or 9 : 3 : 3 : 1 ratio is an expected pattern for many offspring, not a guarantee that every small family will show exact counts.
Quiz
What was central to Mendel’s method?
Which genotypes are tall when T is dominant?
What is the genotype ratio in Tt × Tt?
Which gametes can RrYy form?
What demonstrates independent inheritance?
Practice Problems
- Problem: Cross TT with tt. Solution: Gametes are T and t, so every offspring is Tt and tall.
- Problem: Cross Tt with tt. Solution: Gametes are T/t and t; offspring are Tt and tt in a 1 : 1 genotype and phenotype ratio.
- Problem: Two tall plants produce a short offspring. Identify both genotypes. Solution: The short offspring is tt and received t from each parent, so both tall parents must be Tt.
- Problem: List gametes from RrYy and state the rule used. Solution: RY, Ry, rY and ry; each gamete receives one allele from each gene pair.
- Problem: In RrYy × RrYy, calculate the probability of round green offspring. Solution: Round is 3/4 and green yy is 1/4; independent multiplication gives 3/16, matching the second group in 9 : 3 : 3 : 1.
Key Takeaways
• Each sexually produced offspring receives one gene copy from each parent. • Mendel used controlled crosses, contrasting traits and numerical counts. • A dominant allele is expressed in a heterozygous combination; a recessive phenotype requires two recessive copies. • Genotype describes allele combination, while phenotype describes observable expression. • A Tt × Tt cross gives an expected genotype ratio of 1 : 2 : 1 and phenotype ratio of 3 : 1. • Recessive information can remain present while hidden in a heterozygote. • Independent inheritance creates new combinations and an expected 9 : 3 : 3 : 1 dihybrid ratio. • Expected ratios become clearer when many offspring are counted.