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A new generation inherits a common basic body design and small differences from the previous one, and also adds new differences of its own. In asexual reproduction, for example in bacteria, the offspring are very similar and only minor differences arise from small inaccuracies in DNA copying. In sexual reproduction far greater diversity is generated.
Not all variations have equal chances of surviving. Different individuals have different advantages. For example, bacteria that can withstand heat survive better in a heat wave. The selection of variants by environmental factors forms the basis of evolution.
In an asexually reproducing population, if trait A exists in 10% and trait B in 60%, trait B is likely to have arisen earlier, because it has had more time to spread through successive generations.
Heredity is the transmission of characteristics from parents to offspring. A child bears the basic features of a human being but does not look exactly like its parents. Both parents contribute almost equal amounts of genetic material, so for each trait the child has two versions, one from each parent.
Free and attached earlobes are two variants found in human populations. Comparing students with their parents lets us look for a rule for how the trait is inherited.
Gregor Johann Mendel (1822–1884) was the first to keep count of individuals showing a trait in each generation and combined this with mathematics. He worked on garden peas, using contrasting characters: tall/short plants, round/wrinkled seeds, white/violet flowers.
Mendel proposed that two copies of a factor (now called a gene) control a trait. The two copies may be identical or different. A single copy of T is enough to make the plant tall, while both copies must be t for it to be short. So TT and Tt are tall and only tt is short.
To confirm that F2 had the ratio 1 : 2 : 1, one would let each F2 plant self-pollinate and examine its progeny: TT plants give only tall; Tt plants give tall and short in the ratio 3 : 1; tt plants give only short.
Crossing a tall plant with round seeds and a short plant with wrinkled seeds gives F1 plants that are all tall with round seeds, so tallness and round seeds are dominant. Self-pollination of F1 gives F2 plants of four kinds, including new combinations such as tall with wrinkled seeds and short with round seeds. The ratio is 9 : 3 : 3 : 1. Hence the traits are inherited independently.
Round yellow (RRYY) and wrinkled green (rryy) peas: F1 all round yellow (RrYy). F2 counts of 556 seeds: 315 round yellow, 108 round green, 101 wrinkled yellow and 32 wrinkled green, close to the ratio 9 : 3 : 3 : 1.
DNA is the information source for making proteins in the cell. A section of DNA that gives information for one protein is a gene. Proteins control characteristics. Take tallness: plant height depends on the amount of a growth hormone, which depends on how efficiently an enzyme works. If the gene for the enzyme is altered and the enzyme works less efficiently, less hormone is made and the plant is short. So genes control traits.
Each cell has two copies of every gene, one from each parent. For this to work, each germ cell (gamete) must have only one copy. The DNA is not a single long thread but is in separate pieces, the chromosomes. Each cell has two copies of each chromosome, one from each parent. A germ cell takes one chromosome of each pair, either maternal or paternal in origin; this makes independent inheritance possible. When two germ cells fuse, the normal number is restored and the stability of the DNA of the species is ensured.
Both the male and the female germ cells carry only half the number of chromosomes (one from each pair). On fertilisation they combine, so the progeny gets exactly half its genes from each parent.
Humans have 22 pairs of ordinary chromosomes plus one pair of sex chromosomes. Women have a perfect pair, XX. Men have a mismatched pair: a normal X and a short Y, so men are XY. All eggs have an X. Half the sperms carry X and half carry Y. A child who gets an X from the father is a girl (XX) and one who gets a Y is a boy (XY). The father's chromosome decides the sex, and the chance of a boy or girl is equal.
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Trait B. In asexual reproduction the variations are copied through successive generations; a trait present in 60% of the population has been inherited by many more individuals, so it has had more time to spread. Trait A, in only 10%, is more recent.
Variations give different individuals different advantages. If the environment changes, some variants may be better suited and survive and reproduce (for example, heat-resistant bacteria in a heat wave), so the species survives even if others die. Selection of such variants by the environment is the basis of evolution.
When Mendel crossed tall and short pea plants, all F1 plants were tall: only one parental trait was expressed (the dominant, tallness). When F1 plants self-pollinated, one quarter of the F2 plants were short: the shortness trait had been hidden (recessive) in F1 and reappeared when both copies were t. Hence traits may be dominant or recessive.
In a dihybrid cross of tall round-seeded plants with short wrinkled-seeded plants, F1 plants were all tall and round. In F2, in addition to the parental types (tall round, short wrinkled), new combinations appeared (tall wrinkled and short round) in the ratio 9 : 3 : 3 : 1. This shows that plant height and seed shape are inherited independently of each other.
As given, the information is not considered enough to settle dominance: we do not know the genetic make-up (the copies of the gene) of the parents. The O daughter has an O version from each parent, so the father must carry one O version in addition to the one that gives him group A. If group A is expressed in him despite carrying O, A would be dominant over O, but this needs knowing his make-up and the way blood groups are inherited (there are more than two versions, A, B and O), which is beyond the simple two-version rule of this chapter.
By the chromosomes inherited from the father. Women have XX and men have XY. All eggs carry an X; half of the sperms carry X and half carry Y. If an X sperm fertilises the egg the child is a girl (XX); if a Y sperm does, the child is a boy (XY). So the chances of a boy or a girl are equal.
Half the progeny are short, so the tall parent has one t: Tt (crossed with tt gives Tt and tt in equal numbers). All the progeny are violet: the tall parent must be WW (WW × ww gives all Ww, violet).
Answer: (c) TtWW
No. The finding only shows that the trait runs in families. Both parents with light eyes having light-eyed children is consistent with the trait being dominant or recessive; we would need to know the eye colours of the grandparents and of many families, and the genetic make-up of the parents, to decide which is dominant.
Select pure-breeding dogs of two different coat colours, for example black and brown.
Cross a black dog with a brown dog and note the coat colour of all the puppies (the F1).
If all the F1 puppies are of one colour, that colour is dominant (the other is recessive).
Cross two F1 dogs and count the F2 puppies of each colour; a ratio close to 3 : 1 confirms it.
Repeat with many litters to get a reliable result.
The DNA is in separate chromosomes present in pairs in body cells, one of each pair from each parent. The germ cells (egg and sperm) are formed with only one chromosome from each pair, that is, half the number. At fertilisation the nuclei fuse, and the zygote gets one set from the father and one from the mother, equal in number.