0610 · 17.4Monohybrid inheritance

Alleles, genotype and dominance

0:00Two plants have purple flowers, but when they reproduce, some of their offspring have white flowers. How can a characteristic appear when neither parent shows it? The answer is that the features we can see do not reveal every allele an organism carries. Inheritance becomes much easier to understand once we separate the genetic information from its visible effects. In this video, we will follow alleles from parents into offspring and use genetic diagrams to predict the results.

0:25We will also work backwards from family trees, identify an unknown genotype, and explain blood groups and red-green colour blindness. Throughout, the important question is not just what the answer is, but why that answer follows from the alleles. First, we need a small amount of background. A gene is a length of DNA that codes for a protein, and alleles are alternative forms of a gene. For the examples we are studying, an organism normally has two alleles of a particular gene, one inherited from each parent.

0:55These occupy corresponding positions on a matching pair of chromosomes. During meiosis, these alleles separate, so each gamete receives just one allele of the pair. At fertilisation, two gametes join, restoring a pair of alleles in the offspring. Inheritance is the transmission of genetic information from generation to generation. A monohybrid cross follows the inheritance of one gene, rather than several genes at once. Two terms organise everything that follows.

1:21Genotype is the genetic make-up of an organism, described in terms of the alleles present. Phenotype is the observable features of an organism. For our flower example, the genotype tells us which flower-colour alleles a plant carries, while the phenotype tells us whether its flowers are purple or white. Suppose the allele for purple flowers is dominant and the allele for white flowers is recessive. A dominant allele is an allele that is expressed if it is present in the genotype.

1:47A recessive allele is an allele that is only expressed when there is no dominant allele of the gene present in the genotype. Therefore, a plant carrying both alleles has purple flowers. We represent these alleles using the same letter, with a capital letter for the dominant allele and a lower-case letter for the recessive allele. On screen, capital P represents purple and lower-case p represents white. Two capital letters produce purple flowers, and one capital with one lower-case letter also produces purple flowers.

2:17Only two lower-case letters produce white flowers. Homozygous means having two identical alleles of a particular gene. Our plant could be homozygous dominant, carrying two purple alleles, or homozygous recessive, carrying two white alleles. Heterozygous means having two different alleles of a particular gene. The heterozygous plant carries one purple allele and one white allele, even though only the purple phenotype is visible. Notice that dominance describes expression, not strength, quality or frequency.

2:47A dominant allele is not necessarily common, and a recessive allele is not necessarily rare. Also, the recessive allele has not disappeared from the heterozygous plant. It remains part of the genotype and can still be passed to offspring. Two identical homozygous individuals that breed together will be pure-breeding for that characteristic. Two homozygous purple plants can each pass on only a purple allele, so all their offspring are homozygous purple.

3:13Two homozygous white plants similarly produce only homozygous white offspring. Pure-breeding therefore describes a consistent inheritance outcome, not simply two parents that happen to look alike. A heterozygous individual will not be pure-breeding for that characteristic because it can produce gametes carrying different alleles. Depending on the other parent, its offspring can have different genotypes and phenotypes. However, crossing a homozygous purple plant with a homozygous white plant gives all heterozygous purple offspring.

3:41All offspring look alike in that cross, but they are not themselves pure-breeding.

Punnett squares for monohybrid crosses

3:45Now let us solve the mystery from the beginning. Suppose both purple parents are heterozygous, each carrying one purple allele and one white allele. We start a genetic diagram by writing their phenotypes and genotypes. Next, we show their possible gametes, because it is the gametes, not the parents’ complete allele pairs, that combine during fertilisation. Each parent produces two possible types of gamete, one carrying the purple allele and one carrying the white allele.

4:12In this cross, the two types are equally likely. Put one parent’s gametes across the top of a Punnett square and the other parent’s down the left side. Each box then combines the allele above it with the allele beside it. The first combination contains two purple alleles, so it is homozygous dominant. Two combinations contain one purple allele and one white allele, so they are heterozygous. The final combination contains two white alleles, so it is homozygous recessive.

4:38The genotypic ratio is therefore one homozygous dominant to two heterozygous to one homozygous recessive. Now translate those genotypes into phenotypes. The homozygous dominant offspring and both heterozygous combinations have purple flowers because they contain the dominant allele. Only the homozygous recessive combination has white flowers. The phenotypic ratio is three purple to one white, and the probability of white offspring is one in four, or twenty-five per cent. A Punnett square shows possible outcomes and their probabilities, not a schedule for births.

5:13Four boxes do not mean that every four offspring must include exactly one white individual. Each fertilisation is a new chance combination of gametes. Across many offspring, the proportions are expected to approach the predicted ratio, but a small group may differ substantially. Change the cross to a heterozygous purple plant and a homozygous white plant. The purple plant produces gametes carrying either allele, while the white plant produces gametes carrying only the white allele.

5:39Half the combinations are heterozygous and half are homozygous recessive. Therefore, the phenotypic ratio is one purple to one white, with a fifty per cent probability of each. This gives us a reliable method for any straightforward monohybrid cross. Identify the allele key, write the parental genotypes, show the possible gametes, and combine one allele from each parent. Then determine the phenotype of every offspring genotype before calculating the ratio.

6:07Do not automatically write three to one simply because the question mentions a dominant allele.

Test crosses and pedigrees

6:12We can also use a cross to investigate a purple plant whose genotype is unknown. Its phenotype tells us that it has at least one dominant allele, but not whether it is homozygous or heterozygous. A test cross involves breeding this individual with a homozygous recessive individual. The recessive parent supplies only recessive alleles, allowing us to detect what the unknown parent contributes. If the unknown plant is homozygous dominant, all offspring will be heterozygous and purple.

6:40If it is heterozygous, the expected ratio is one purple to one white. Any white offspring show that the unknown parent supplied a recessive allele, proving that it is heterozygous. All purple offspring support a homozygous dominant genotype, but a small sample cannot rule out a heterozygous parent by chance. Sometimes, rather than arranging a cross, we interpret inheritance from a pedigree diagram. This is a family tree showing which individuals have a particular characteristic.

7:09Squares usually represent males, circles represent females, and shading identifies individuals with the characteristic. Always read the key, because shading does not automatically mean that an allele is dominant. Consider a pedigree for a recessive characteristic that is not sex-linked. Two parents without the characteristic have a child who shows it. The affected child must have two recessive alleles, receiving one from each parent. Since neither parent shows the characteristic, both must also have a dominant allele, making both parents heterozygous.

7:41This is the same reasoning as the purple-flower cross, just presented as a family tree. An unaffected individual could be homozygous dominant or heterozygous, so appearance alone may not identify the genotype. Use the alleles required by parents and offspring to narrow the possibilities. A small pedigree may leave some genotypes uncertain, so do not claim more than the evidence supports.

Codominance and blood groups

8:03So far, one allele has dominated the other, but that is not the only relationship possible. Codominance is a situation in which both alleles in heterozygous organisms contribute to the phenotype. Neither allele’s contribution is hidden. Human blood groups provide an important example, because the alleles for blood groups A and B can both be expressed. The blood-group gene has three alleles. We will call them allele A, allele B and allele O, and in genetic diagrams they are written as a capital letter I with a small A, B or O beside it.

8:35There are three alleles in the population, but each person still has only two alleles of this gene. Allele A and allele B are codominant with each other, and both are dominant to allele O. The four possible phenotypes are blood groups A, B, AB and O. Blood group A can result from two A alleles or from allele A with allele O. Blood group B can result from two B alleles or from allele B with allele O.

9:02Blood group AB has allele A with allele B, and blood group O has two copies of the O allele. In group AB, both A and B antigens are present on the red blood cells, rather than blending into a new antigen. Let us cross a group A parent carrying alleles A and O with a group B parent carrying alleles B and O. The first parent produces gametes carrying allele A or allele O, while the second produces gametes carrying allele B or allele O.

9:28The four combinations give blood groups AB, A, B and O. Each has a chance of one in four, giving a phenotypic ratio of one to one to one to one.

Sex-linked colour blindness

9:37Another important pattern is sex linkage. A sex-linked characteristic is a feature in which the gene responsible is located on a sex chromosome, making the characteristic more common in one sex than in the other. In the usual chromosome pattern, females have two X chromosomes and males have one X and one Y chromosome. Red-green colour blindness is an example of an X-linked recessive characteristic. The allele responsible is on the X chromosome, and the Y chromosome does not carry a corresponding allele for this gene.

10:06A male therefore expresses the recessive colour-blindness allele if it is present on his only X chromosome. A female generally needs the recessive allele on both X chromosomes to be affected. This is why males are affected far more often than females. A heterozygous female has one allele for normal colour vision and one allele for red-green colour blindness. She has normal colour vision but can pass on the recessive allele, so she is called a carrier.

10:31For the diagram, we attach the allele symbol to its X chromosome. Capital N represents normal colour vision, while lower-case n represents red-green colour blindness. Cross a carrier mother with a father who has normal colour vision. Her eggs carry either an X chromosome with the normal allele or an X chromosome with the colour-blindness allele. His sperm carry either an X chromosome with the normal allele or a Y chromosome.

10:54Combining these gives a normal-vision daughter, a normal-vision son, a carrier daughter and a colour-blind son. Each combination has a probability of one quarter. Counting colour vision alone, the phenotypic ratio is three with normal colour vision to one with red-green colour blindness. Among sons, however, the ratio is one with normal colour vision to one with colour blindness. This distinction matters because a probability among all children is different from a probability among sons only.

11:23Notice that fathers pass their Y chromosome to sons and their X chromosome to daughters. Therefore, a son cannot inherit this X-linked allele directly from his father. Also, the carrier daughter belongs in the normal-vision phenotype group, even though her genotype differs from her sister’s. Genotype categories and phenotype categories must not be treated as interchangeable. Here is an exam-style question using that cross. Explain why red-green colour blindness is more common in males, then state the probability that a child of these parents is a colour-blind son.

11:55First identify where the gene is located, then explain the difference between males and females. Finally, use the completed Punnett square and check whether the question refers to all children or only sons. The gene responsible is located on the X chromosome. Males have only one X chromosome, so a recessive allele on that chromosome is expressed. Females require the recessive allele on both X chromosomes to show the characteristic.

12:20One of the four equally likely offspring combinations is a colour-blind son, so the probability is one quarter.

Common mistakes and recap

12:27Across all these questions, three mistakes cause particular trouble. Giving a gamete two alleles of the same gene ignores the separation of alleles during meiosis. Deciding that every dominant-looking organism is homozygous ignores the possibility of heterozygosity. Treating predicted ratios as guaranteed family sizes ignores the role of chance in which gametes meet. Let us finish with the connecting idea. Inheritance transmits genetic information, gametes carry one allele of each gene, and fertilisation brings alleles together.

12:55Genotype describes the alleles present, while phenotype describes the observable features. Dominance, codominance and sex linkage explain how those inherited alleles contribute to the phenotype. A Punnett square makes the possible combinations visible, and a pedigree lets us reason backwards from family relationships. A test cross reveals information about an unknown genotype by pairing it with a homozygous recessive individual. Whether you are studying flowers, blood groups or colour vision, follow the alleles first.

13:25The ratios then become something you can explain, rather than something you have to guess.