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Inheritance: Class 12 Biology
Mendel's experiments and laws, monohybrid and dihybrid crosses, test cross, incomplete dominance, codominance and multiple alleles, sex determination, sex linkage, linkage and crossing over, and mutations.
Key points
Genetics is the study of heredity and variation. Gregor Mendel worked on garden pea plants because they grow quickly, have clear contrasting characters, and can be self pollinated or cross pollinated by hand. He studied seven characters, such as tall and dwarf stems, and counted the offspring of many crosses.
A gene is a unit of inheritance, a length of DNA. Alleles are different forms of a gene. A dominant allele shows its effect even when one copy is present, and a recessive allele shows only when two copies are present. Genotype is the genetic make up (TT, Tt, tt), and phenotype is the visible character. An organism with two identical alleles is homozygous, with two different alleles it is heterozygous.
A monohybrid cross follows one character. Pure tall (TT) crossed with pure dwarf (tt) gives all tall F₁ plants (Tt). Self crossing the F₁ gives an F₂ with genotype ratio 1 TT : 2 Tt : 1 tt and phenotype ratio 3 tall : 1 dwarf. This led to the law of segregation: the two alleles of a gene separate during gamete formation, so each gamete gets only one allele.
A test cross is the cross of an organism of dominant phenotype with a homozygous recessive. If the unknown is TT all offspring are tall. If it is Tt, half are tall and half are dwarf (1 : 1). A back cross is a cross of the F₁ with either parent.
A dihybrid cross follows two characters at once. Round yellow (RRYY) crossed with wrinkled green (rryy) gives all round yellow F₁ (RrYy). The F₁ makes four kinds of gamete (RY, Ry, rY, ry) in equal numbers, and the F₂ shows the phenotype ratio 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. This gives the law of independent assortment: alleles of different genes assort independently, when the genes are on different chromosomes.
Not all inheritance is simple dominance. In incomplete dominance the heterozygote is in between: red crossed with white snapdragon gives pink, and the F₂ ratio is 1 red : 2 pink : 1 white. In codominance both alleles show fully. In the ABO blood groups the alleles are Iᴬ, Iᴮ and i, so there are three alleles (multiple alleles) but each person has only two. Iᴬ and Iᴮ are codominant, and both are dominant to i.
ABO genotypes and phenotypes:
- Group A: IᴬIᴬ or Iᴬi.
- Group B: IᴮIᴮ or Iᴮi.
- Group AB: IᴬIᴮ.
- Group O: ii.
- The Rhesus factor is a separate gene: Rh positive is dominant to Rh negative.
Sex determination in humans: there are 22 pairs of autosomes and one pair of sex chromosomes. Females are XX and males are XY. All eggs carry X, while sperm carry X or Y in equal numbers. The sex of a child depends on the sperm, and the chance of a boy or a girl is 50 percent each.
Sex linked genes lie on the X chromosome. Haemophilia and red green colour blindness are recessive X linked conditions. Males have only one X, so one recessive allele is enough to show the trait, while females need two. A carrier mother (XᴴXʰ) and normal father (XᴴY) give daughters that are normal or carriers and sons with a 50 percent chance of being affected. An affected father passes the allele to all his daughters but to none of his sons.
Linkage: genes on the same chromosome tend to be inherited together and do not assort independently. Crossing over during meiosis I, when parts of homologous chromatids are exchanged, makes new combinations (recombinants). The closer two genes lie, the less often they separate. Morgan studied linkage in the fruit fly Drosophila.
A mutation is a sudden change in DNA or chromosomes. Gene mutations are changes in the base sequence, for example sickle cell anaemia is caused by one base change that replaces one amino acid in haemoglobin. Chromosome mutations include Down syndrome, where a person has three copies of chromosome 21 (47 chromosomes in total). Mutagens such as X rays, UV light and some chemicals raise the mutation rate.
Study tip: Always show a key, the parental genotypes, the gametes, a Punnett square and the ratios in every cross, as marks are given for each step. Practise 9 : 3 : 3 : 1, test cross, blood group and X linked questions until you can do them quickly.
Definitions and flashcards
Say the meaning first, then open the card to check.
Allele
One of the alternative forms of a gene.
Genotype
The genetic make up of an organism.
Phenotype
The observable characteristics of an organism.
Homozygous
Having two identical alleles of a gene.
Test cross
A cross of an individual of unknown genotype with a homozygous recessive.
Codominance
A condition in which both alleles of a heterozygote are fully expressed.
Sex linkage
Inheritance of a gene that lies on a sex chromosome.
Crossing over
The exchange of segments between non-sister chromatids of homologous chromosomes.
Mutation
A sudden random change in the genetic material.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
State Mendel's law of segregation. (2 marks)
The two alleles that control a character separate from each other during gamete formation, so each gamete carries only one allele of each gene.
What is a test cross and why is it used? (2 marks)
It is a cross between an organism showing the dominant trait and a homozygous recessive. It is used to find whether the dominant organism is homozygous or heterozygous: all offspring dominant means homozygous, and a 1 : 1 ratio means heterozygous.
Give the genotypes of the parents and offspring when a heterozygous tall pea plant is crossed with a dwarf plant. (3 marks)
The parents are Tt and tt. The gametes of Tt are T and t, and those of tt are all t. The offspring are Tt (tall) and tt (dwarf) in the ratio 1 : 1, so 50 percent are tall and 50 percent are dwarf.
Explain why haemophilia is more common in males. (3 marks)
The gene lies on the X chromosome and the allele for haemophilia is recessive. A male has one X and a Y that carries no matching allele, so one recessive allele makes him affected. A female has two X chromosomes and must inherit the recessive allele on both to be affected, otherwise she is only a carrier. So affected males are much more common.
Explain how the sex of a child is decided. (3 marks)
A woman has chromosomes XX, so every egg has one X. A man has XY, so half of the sperm carry X and half carry Y. If an X sperm fertilises the egg the child is a girl (XX). If a Y sperm fertilises it, the child is a boy (XY). So the father's sperm decides the sex.
Explain the inheritance of ABO blood groups. Can parents of groups A and B have a child of group O? (5 marks)
The blood group is controlled by one gene with three alleles, Iᴬ, Iᴮ and i. Iᴬ and Iᴮ are codominant and both are dominant to i. Each person inherits two alleles, one from each parent, which gives groups A (IᴬIᴬ, Iᴬi), B (IᴮIᴮ, Iᴮi), AB (IᴬIᴮ) and O (ii). Yes, parents of groups A and B can have a child of group O if the mother is Iᴬi and the father is Iᴮi. Each gives the allele i, so the child is ii with a 25 percent chance. The other children can be Iᴬi (A), Iᴮi (B) or IᴬIᴮ (AB).
Common mistakes
- Writing the F₂ ratio of a monohybrid cross as 3 : 1 for the genotype. The genotype ratio is 1 : 2 : 1 and the phenotype ratio is 3 : 1.
- Showing alleles of one gene in a gamete as a pair. A gamete has only one allele of each gene.
- Saying Y carries genes for colour blindness, or that sons inherit an X linked allele from their father.
- Treating codominance and incomplete dominance as the same. In codominance both traits show, in incomplete dominance a blend appears.
- Forgetting to write the key (symbols for alleles) and the gametes in a genetic cross.