Class 10 Science · Chapter 8 NotesHeredity

Reproduction produces new individuals that are similar to their parents but not identical. In asexual reproduction, differences are very small and arise mainly from minor inaccuracies in DNA copying. In sexual reproduction, the combining of genetic material from two parents creates much greater diversity. These variations are inherited by the next generation, and each new generation adds its own newly created differences. Not all variations are equally useful. Depending on their nature, different individuals may have different advantages in a given environment. For example, bacteria that can withstand heat survive better in a heat wave. Environmental factors select variants that are better suited to survive, and this selection forms the basis of evolutionary processes.

• Asexual reproductionsmall variations, mainly from DNA copying errors
• Sexual reproductionlarger diversity because two parents contribute genetic material
• Selection of variants by environmental factors is the basis of evolution

The rules of heredity decide how traits and characteristics are reliably passed on from one generation to the next. A child shows all the basic features of a human being but does not look exactly like either parent, and human populations show a great deal of variation. Traits such as free or attached earlobes are inherited from parents. Since both the father and the mother contribute practically equal amounts of genetic material to the child, each trait can be influenced by both paternal and maternal DNA. This means that for each trait there are two versions in every child, and the trait that is actually seen depends on the rules of inheritance.

• Hereditythe passing of traits from parents to offspring
• Both parents contribute almost equal amounts of genetic material
• Each trait exists in two versions in a sexually reproducing organism

Gregor Johann Mendel studied contrasting visible characters of garden peas, such as round or wrinkled seeds, tall or short plants, and white or violet flowers. He crossed a tall plant with a short plant and found that all the first-generation, or F1, progeny were tall; there were no medium-height plants. This showed that only one parental trait was expressed, not a mixture. When the F1 tall plants were self-pollinated, the second-generation, or F2, progeny were not all tall: about one quarter of them were short. This showed that both tallness and shortness were inherited in the F1 plants, but only tallness was expressed. Mendel therefore proposed that two copies of a factor, now called a gene, control each trait in sexually reproducing organisms.

• F1 generationall tall, no halfway characteristics
• F2 generationtall and short plants appear, with short plants making up about one quarter
• Two copies of a factor (gene) control each trait

In Mendel's explanation, both TT and Tt plants are tall, while only tt plants are short. This means a single copy of T is enough to make the plant tall, but both copies must be t for the plant to be short. Traits like T are called dominant traits, and traits that behave like t are called recessive traits. When pea plants showing two different characteristics are bred, the F1 progeny of a tall plant with round seeds and a short plant with wrinkled seeds are all tall with round seeds, so tallness and round seeds are dominant. When these F1 plants are self-pollinated, the F2 progeny include tall plants with round seeds, short plants with wrinkled seeds, and also new combinations such as tall plants with wrinkled seeds and short plants with round seeds. This shows that the tall/short trait and the round/wrinkled seed trait are inherited independently.

• Dominant traitexpressed even when only one copy is present
• Recessive traitexpressed only when both copies are the same recessive form
• Two separate traits are inherited independently, producing new combinations in the F2 generation

Cellular DNA is the information source for making proteins in the cell. A section of DNA that provides information for one protein is called the gene for that protein. Proteins control the characteristics of an organism. For example, plant height depends on the amount of a particular plant hormone, and the amount of hormone made depends on the efficiency of the process that produces it. An enzyme is important in this process. If the enzyme works efficiently, a lot of hormone is made and the plant grows tall. If the gene for that enzyme is altered so that the enzyme becomes less efficient, less hormone is made and the plant remains short. In this way, genes control characteristics or traits.

• Genea section of DNA that provides information for one protein
• Proteins control the traits of an organism
• A change in a gene can change the efficiency of an enzyme and therefore the trait

If both parents contribute equally to the DNA of the progeny, each parent must contribute one copy of the same gene, so each pea plant must have two sets of all genes. For this to work, each germ cell must carry only one set of genes. This is possible because each gene set is not one long thread of DNA but is present as separate independent pieces called chromosomes. Each body cell has two copies of each chromosome, one from each parent. Every germ cell takes one chromosome from each pair, which may be of maternal or paternal origin. When two germ cells combine, the normal number of chromosomes is restored in the progeny, ensuring the stability of the DNA of the species. This mechanism explains Mendel's results and is used by all sexually reproducing organisms.

• Chromosomea separate independent piece of DNA carrying many genes
• Body cells have two copies of each chromosome; germ cells have one
• Fusion of two germ cells restores the normal chromosome number in the offspring

Different species determine sex in different ways. In some reptiles, the temperature at which fertilised eggs are kept decides whether the young ones are male or female. In snails, individuals can change sex, showing that sex is not genetically determined in them. In human beings, sex is largely genetically determined. Human beings have 22 pairs of chromosomes in which each chromosome has a maternal and a paternal copy. The remaining pair, called the sex chromosomes, is not always a perfect pair. Women have a perfect pair of sex chromosomes, both called X, so they are XX. Men have a mismatched pair with one normal-sized X and one short Y, so they are XY. All children inherit an X chromosome from their mother. A child who inherits an X chromosome from the father is a girl, and one who inherits a Y chromosome from the father is a boy. Thus, the sex of the child is determined by what is inherited from the father.

• WomenXX; Men: XY
• Mother always contributes an X chromosome
• Father's contribution decides the sex of the child: X for a girl, Y for a boy