ACCUMULATION OF VARIATION DURING REPRODUCTION

1. Inheritance and Variation: Inheritance refers to the passing of genetic information from one generation to the next. This inheritance provides a common basic body design for a species, but it also introduces subtle changes or variations in the genetic makeup of each new generation.
2. Reproduction and Diversity: When a new generation reproduces, it inherits both the common traits from the previous generation and introduces new variations. In asexual reproduction (e.g., bacterial division), the resulting individuals are very similar, with minor differences due to DNA copying errors. In sexual reproduction, even greater diversity is generated due to the combination of genetic material from two parents.
3. Survival and Selection: Not all variations have equal chances of survival in a given environment. The nature of the variations determines the advantages or disadvantages of different individuals. For example, in a heatwave, bacteria that can withstand high temperatures may have a better chance of survival. This natural selection of variants by environmental factors forms the basis for evolutionary processes.
4. Evolution: Evolution is the result of the accumulation of advantageous variations over time. Those individuals with traits that enhance their survival and reproductive success are more likely to pass on their genes to the next generation, leading to the gradual change and adaptation of species to their environments.

HEREDITY

The passage discusses the rules of heredity and how traits are inherited from parents. It also introduces the contributions of Gregor Mendel in understanding the inheritance of traits, specifically using the example of garden pea plants. Here are the key points:

1. Inherited Traits: Inherited traits are characteristics passed from parents to their offspring. These traits can result in both similarities and differences among individuals within a species.
2. Equal Genetic Contribution: In humans and many other sexually reproducing organisms, both the father and the mother contribute genetic material to their offspring. As a result, each trait can be influenced by genetic information from both parents.
3. Mendel’s Experiments: Gregor Mendel conducted experiments using garden pea plants with contrasting visible traits, such as tall and short plants. He crossed pea plants with different traits and studied the inheritance of these traits in the progeny.
4. First-Generation (F1) Progeny: Mendel’s experiments in the first generation (F1) showed that there were no “intermediate” or mixed traits. For example, when tall and short plants were crossed, all the F1 progeny were tall, indicating the dominance of one trait.
5. Second-Generation (F2) Progeny: To further understand the inheritance of traits, Mendel allowed the F1 tall plants to reproduce by self-pollination. In the second generation (F2), Mendel observed that one-quarter of the progeny were short, while three-quarters were tall.
6. Two Copies of Genes: Mendel proposed that organisms have two copies of factors (now called genes) controlling traits. These copies can be identical or different, depending on the parental genetic makeup.
7. Dominant and Recessive Traits: Mendel’s work led to the concept of dominant and recessive traits. Traits like ‘T’ (for tallness) are considered dominant, and a single copy of the dominant gene is sufficient to express the trait. Traits like ‘t’ (for shortness) are considered recessive and are expressed only when an individual has two copies of the recessive gene.

What happens when pea plants showing two different characteristics, rather than just one, are bred with each other?

What does the progeny of a tall plant with round seeds and a short plant with wrinkled seeds look like?

1. Breeding Pea Plants with Different Characteristics: When pea plants with two different characteristics (e.g., tall and short plants, round and wrinkled seeds) are bred with each other, the characteristics of the first generation (F1) progeny are influenced by dominant and recessive traits.
2. Dominant and Recessive Traits: In the case of tall plants and round seeds, these traits are dominant. So, when a tall plant with round seeds is crossed with a short plant with wrinkled seeds, all the F1 progeny are tall and have round seeds. This indicates that tallness and round seeds are dominant traits.
3. F2 Progeny: When the F1 progeny are used to generate the second-generation (F2) progeny by self-pollination, a Mendelian experiment reveals that the F2 progeny exhibit various combinations of traits. Some are tall with round seeds, some are short with wrinkled seeds, but there are also new combinations.
4. Independent Inheritance: The key insight is that traits such as tall/short and round seed/wrinkled seed are inherited independently of each other. This means that the genetic factors (genes) that control height and seed shape segregate independently during reproduction, leading to various combinations of traits in the F2 offspring.
5. Recombination of Genetic Factors: The formation of new combinations of traits in the F2 generation occurs because the factors controlling seed shape and seed color recombine when the gametes (sperm and egg cells) fuse to form zygotes. This recombination leads to the inheritance of new combinations of traits in the F2 progeny.

How do these Traits get Expressed?

1. Genes and Traits: Genes are sections of cellular DNA that provide information for making proteins. Each gene is responsible for a specific protein, which in turn can control certain characteristics or traits in an organism. For example, the amount of a plant hormone may depend on the efficiency of an enzyme involved in its production.
2. Gene Variations: Variations in genes can result in differences in traits. If a gene that codes for an enzyme involved in hormone production is altered and becomes less efficient, it can lead to reduced hormone production and, consequently, a shorter plant.
3. Genetic Contributions from Both Parents: In sexual reproduction, both parents contribute genetic material to the progeny. Each pea plant, for example, has two sets of genes, one inherited from each parent. This implies that each germ cell (sperm and egg) must carry only one set of genes.
4. Independently Inherited Traits: The passage explains that genes are present as separate, independent pieces known as chromosomes. Each cell has two copies of each chromosome, one from each parent. Germ cells, however, carry only one chromosome from each pair. When two germ cells combine during fertilization, they restore the normal number of chromosomes in the progeny. This mechanism ensures that traits, such as height and seed shape, are independently inherited.
5. Mechanism of Inheritance in Asexual Reproduction: The passage hints that asexual reproduction organisms also follow similar rules of inheritance but doesn’t provide specific details. In asexual reproduction, offspring are typically genetically identical to the parent, as there is no genetic recombination between two parents as in sexual reproduction.

Sex Determination

1. Sex Determination Strategies: Different species use various strategies to determine the sex of newborn individuals. Some rely on environmental cues, like temperature, to determine sex, while others can change sex during their lifetime. In the case of human beings, sex is largely genetically determined.
2. Genetic Inheritance: In humans, the sex of an individual is determined by the genes inherited from their parents. However, the question arises: if both parents contribute similar gene sets, how does genetic inheritance determine sex?
3. Sex Chromosomes: The explanation lies in the fact that not all human chromosomes are paired. Most human chromosomes come in maternal and paternal pairs (22 pairs), but one pair, known as the sex chromosomes, is different. Women have two X chromosomes (XX), forming a perfect pair, while men have an unmatched pair, with one X and one Y chromosome (XY).
4. Inheritance Pattern: As shown in Figure 8.6, the sex of children is determined by what they inherit from their father. All children inherit an X chromosome from their mother. If a child inherits an X chromosome from their father, they will be a girl. If they inherit a Y chromosome from their father, they will be a boy.
5. Half Boys and Half Girls: In this genetic inheritance pattern, approximately half of the children will be boys (XY) and the other half will be girls (XX).

In humans, sex determination is based on the presence of specific sex chromosomes. The sex of an individual is determined by the combination of sex chromosomes they inherit from their parents. Here’s how it works:

1. Male (XY) and Female (XX):
   • Males have one X chromosome and one Y chromosome (XY).
   • Females have two X chromosomes (XX).
2. Inheritance Pattern:
   • A child receives one sex chromosome from each parent.
   • The mother always contributes an X chromosome (X) because she has two X chromosomes (XX).
   • The father can contribute either an X or a Y chromosome, as he has one X and one Y chromosome (XY).
   • The combination of sex chromosomes inherited from the parents determines the sex of the offspring.
3. Determining Sex:
   • If a child inherits an X chromosome from their father (XY), they will be male (a boy).
   • If a child inherits an X chromosome from their father (XX), they will be female (a girl).
4. Sex Ratio:
   • The sex ratio among offspring is approximately equal, with about half of the children being boys (XY) and the other half being girls (XX).