Blood Groups & Mendel's Peas

Blood Groups & Mendel's Peas

Genetics did not begin with a microscope. It began in a monastery garden, with pea plants, careful crossing, and above all with somebody bothering to count. This module meets the man who founded the subject without knowing that genes or DNA existed, and then takes on the ABO blood groups, where the familiar dominant-or-recessive rule bends in two directions at once.

Gregor Mendel

In the 1860s Gregor Mendel cross-bred pea plants and, crucially, counted the offspring in their thousands. He crossed a pure tall plant with a pure short one. Every plant in the first generation was tall, and shortness seemed to have vanished. Then he crossed those tall plants with each other, and shortness came back: 787 tall and 277 short, a ratio of very nearly three to one. That ratio is the whole discovery. Shortness had not been diluted or destroyed, so characteristics could not be blending fluids as everyone assumed. They had to be separate units, passed on whole, with one able to mask the other. He had deduced genes and dominance without ever seeing either, and the work was ignored for about thirty-five years.

Reading the ratio

Mendel crossed two tall pea plants and got 787 tall offspring and 277 short ones. Why was the reappearance of short plants so important?

• Shortness had been carried, hidden, by both tall parents, so characteristics must pass on as separate units rather than blending together
• A new mutation for shortness must have appeared in that generation
• It showed that tallness is a weak characteristic
• It showed that growing conditions determine height

The words for it

Five terms, and the last two are the ones this module exists to separate:

Put them together

A version of a gene is called an _____, and each person inherits two of them for a given gene. An allele that shows even when only one copy is present is _____, while one that shows only when both copies match is _____. Where two alleles are both fully expressed together, neither masking the other, they are _____.

Masked, or both shown?

Two alleles meeting in one organism can behave in two quite different ways, and telling them apart is the core of this topic:

Red, white, or pink?

A red-flowered plant is crossed with a white-flowered one. The offspring have flowers with distinct red AND white patches, rather than pink flowers. What does that tell you?

• The alleles are codominant: both are fully expressed, so both colours appear separately rather than being averaged
• The alleles blended, and the blending was incomplete
• Red is dominant over white
• A mutation occurred during the cross

The ABO system

Human blood group is decided by one gene with three alleles in the population: I^A, I^B and i. Any one person carries two of them. Two rules govern how those two behave: • I^A and I^B are codominant. If a person has one of each, both are fully expressed. • i is recessive. It shows only when there is no I^A or I^B to mask it. Those two rules generate every genotype and every group, so there is nothing else to memorise. Work from the rules and the four groups fall out of them.

Work out the group

Cross: I^A i × I^B i

A group-A parent I^A i gives alleles A and o (across the top); a group-B parent I^B i gives B and o (down the side). Fill in the four offspring genotypes. (A = I^A, B = I^B, o = i.)

• AB
• Ao
• Bo
• oo

Which children are possible?

Blood groups can rule possibilities in and out. Work through four cases.

• The cross you just completed was group A × group B, both carrying i. What is striking about the four possible children?
• A group-O child is born to a group-A mother and a group-B father. Is that possible?
• Could a group-AB parent have a group-O child?
• Why can blood groups rule a possibility OUT with certainty, but rarely prove a relationship?

Which are true?

Select ALL THREE statements that are TRUE.

• A gene can have three alleles in the population while any one person carries only two
• Codominance is not a halfway blend: both alleles are expressed in full
• Mendel could deduce that inherited factors are discrete without ever seeing one, because the ratios told him so
• Each person carries all three ABO alleles
• The i allele is dominant, which is why group O is so common
• Codominant alleles mix to produce an intermediate characteristic

Explain the case

Exam practice. In about 50 words, explain how a group-O child can be born to a group-A mother and a group-B father. Include:

• the genotype each parent must have for this to be possible
• which allele each of them passes on to that child
• why the child is group O rather than group A, B or AB

Inheritance summary

Gregor Mendel worked out the rules of inheritance from _____ plants, deducing that characteristics pass on as separate units because a hidden characteristic reappeared in a roughly three-to-one _____. The ABO blood-group gene has three alleles, an example of _____ alleles, of which I^A and I^B are codominant and i is recessive. A person is blood group O only if their genotype is _____.