Monohybrid Inheritance

Genotype and Phenotype

Inheritance is the transmission of genetic information from generation to generation. The genotype of an organism is its genetic make-up, described in terms of the alleles present. The phenotype is its observable features.

In pea plants, one gene controls height. The allele T gives tall plants and the allele t gives dwarf plants. The alleles an individual has are its genotype; whether it is tall or dwarf is its phenotype.

Worked example (genotype and phenotype): A pea plant has the alleles Tt and is tall. State its genotype and its phenotype. Step 1. The genotype is the alleles present: Tt. Step 2. The phenotype is the observable feature: tall. Answer: genotype Tt; phenotype tall.

Worked example (linking genotype to phenotype): A pea plant has the genotype tt. State its phenotype and explain your answer. Step 1. The only alleles present are t, the recessive allele. Step 2. A recessive allele is only expressed when no dominant allele is present, so the plant is dwarf. Answer: dwarf.

Dominant and Recessive Alleles

A dominant allele is expressed if it is present in the genotype. A recessive allele is only expressed when there is no dominant allele present. A capital letter is used for the dominant allele (T) and a small letter for the recessive allele (t).

Two identical homozygous individuals that breed together will be pure-breeding: all their offspring have the same genotype and phenotype as the parents. A heterozygous individual will not be pure-breeding.

Worked example (describing genotypes): Describe the genotypes TT and Tt, and state whether each would be pure-breeding. Step 1. TT has two identical alleles, so it is homozygous and is pure-breeding when bred with another TT. Step 2. Tt has two different alleles, so it is heterozygous and is not pure-breeding. Answer: TT: homozygous, pure-breeding; Tt: heterozygous, not.

Genetic Diagrams

A genetic diagram predicts the results of a cross. Each parent makes gametes containing one allele of the gene. Fertilisation then combines two gametes at random.

The offspring are TT, Tt, Tt and tt. Three of the four genotypes give tall plants and one gives a dwarf plant, so the phenotype ratio is 3 : 1.

Worked example (using a Punnett square): Two heterozygous tall pea plants are crossed. State the chance that an offspring is dwarf. Step 1. The offspring genotypes are TT, Tt, Tt and tt. Step 2. Only tt is dwarf, so the chance is 1 in 4 (25%). Answer: 1 in 4 (25%).

Worked example (predicting a ratio): Two heterozygous tall pea plants are crossed. State the expected phenotype ratio of the offspring. Step 1. The offspring genotypes are TT, Tt, Tt and tt. Step 2. TT, Tt and Tt are tall (3). tt is dwarf (1). Answer: 3 tall : 1 dwarf.

More Crosses

The ratio of phenotypes depends on the genotypes of the parents. In Core you must work out 1 : 1 and 3 : 1 ratios.

Worked example (a 1 : 1 ratio): A heterozygous tall plant is crossed with a dwarf plant. State the expected phenotype ratio. Step 1. The gametes are T and t from the tall plant, and only t from the dwarf plant. Step 2. The offspring are Tt and tt in equal numbers: 1 tall : 1 dwarf. Answer: 1 tall : 1 dwarf.

Worked example (a cross with a homozygous parent): A pure-breeding tall plant (TT) is crossed with a dwarf plant (tt). State the genotypes and the phenotype of the offspring. Step 1. The tall plant can only make T gametes and the dwarf plant can only make t gametes. Step 2. Every offspring receives one T and one t, so all are Tt and all are tall. Answer: all Tt; all tall.

Pedigree Diagrams

A pedigree diagram shows a characteristic across generations. A square is a male, a circle is a female, and shading shows who has the characteristic. A horizontal line joins parents, and a vertical line leads to their children.

Reading a pedigree

Worked example (interpreting a pedigree): In Fig 17.15, explain how you know the characteristic is caused by a recessive allele. Step 1. Individuals I-1 and I-2 do not have the characteristic, but their daughter II-2 does. Step 2. So the allele must be recessive: both parents carry it but do not show it. Answer: recessive; both parents carry it.

Worked example (working out a genotype): In Fig 17.15, state the genotype of II-2, using A for the dominant allele and a for the recessive allele. Step 1. II-2 has the characteristic, which is caused by the recessive allele. Step 2. A recessive allele is only expressed when no dominant allele is present, so II-2 is aa. Answer: aa.

Test Cross and Codominance

ExtendedUsing a test cross

An organism with the dominant phenotype could be homozygous (TT) or heterozygous (Tt). To find out, cross it with a homozygous recessive (tt) and look at the offspring.

Any dwarf offspring show that the unknown parent carries the recessive allele.

ExtendedCodominance

In codominance, both alleles in a heterozygous organism contribute to the phenotype. Neither allele is dominant. Different alleles are written as a capital letter with a superscript, for example CR and CW.

Worked example (a test cross): A tall pea plant is crossed with a dwarf plant. Half of the offspring are dwarf. State the genotype of the tall parent, and explain. Step 1. Half the offspring are dwarf (tt), so each parent gave a t allele. Step 2. So the tall parent is heterozygous, Tt. Answer: Tt (heterozygous).

ABO Blood Groups

ExtendedThe alleles and phenotypes

The ABO blood group is controlled by one gene with three alleles: IA, IB and Io. The phenotypes are blood groups A, B, AB and O.

ExtendedA cross

A parent of group A (IAIo) and a parent of group B (IBIo) can have children of all four groups.

The four offspring genotypes give groups AB, A, B and O in the ratio 1 : 1 : 1 : 1.

Worked example (blood groups): A mother has blood group O and a father has blood group AB. State the possible blood groups of their children. Step 1. The mother is IoIo, so every gamete carries Io. The father gives either IA or IB. Step 2. The children are IAIo (group A) or IBIo (group B). Answer: group A or group B.

Worked example (two parents of group A): Both parents are group A with the genotype IAIo. Can they have a child with group O? Step 1. Each parent makes IA gametes and Io gametes. Step 2. A child receiving Io from both parents is IoIo, which is group O. The chance is 1 in 4. Answer: yes; 1 in 4.

Sex Linkage

ExtendedRed-green colour blindness

A sex-linked characteristic is one where the gene is on a sex chromosome, so the characteristic is more common in one sex than the other. Red-green colour blindness is an example. The allele is recessive and is on the X chromosome. Normal vision is XB and colour blindness is Xb.

ExtendedA genetic diagram

Worked example (explaining sex linkage): Explain why red-green colour blindness is more common in males than in females. Step 1. The allele is recessive and on the X chromosome. Males have only one X chromosome, and the Y chromosome does not carry the gene, so one Xb allele makes a male colour-blind. Step 2. Females have two X chromosomes, so they need two Xb alleles to be colour-blind. Answer: males need one allele; females need two.