HeredityClass 10 Biology Notes
ACCUMULATION OF VARIATION DURING REPRODUCTION
Reproduction creates new individuals that are similar to their parents but also have subtle differences, known as variations. While some variation occurs even in asexual reproduction, it is maximized through sexual reproduction.
Inheritance is the process that passes traits from one generation to the next. It provides a common basic body design from the parent generation, along with small changes. When this new generation reproduces, it passes on both the traits it inherited and any newly created differences.
Variation in Asexual vs. Sexual Reproduction
- Asexual Reproduction: An organism like a bacterium divides to produce two, which then divide again to produce four. These offspring are very similar to each other. The only variations that arise are due to small, accidental errors during DNA copying.
- Sexual Reproduction: This process generates much greater diversity and variation among offspring.
Variation and Survival
Not all variations give an organism an equal chance of survival. The environment plays a crucial role in selecting which variations are beneficial.
- Environmental Factors: The environment selects for variants that are best suited to it. For example, if a heat wave occurs, bacteria that have a variation allowing them to withstand heat will survive better than those that don't.
- Basis for Evolution: This selection of variants by the environment is the fundamental basis for the process of evolution.
HEREDITY
While reproduction creates variation, its most obvious result is still the creation of offspring with a similar design to the parents. The rules that govern how traits and characteristics are reliably passed down are known as the rules of heredity.
Inherited Traits
When we talk about similarities and differences, we mean the specific characteristics, or traits, that are passed down. A child has the basic features of a human being but doesn't look exactly like their parents because of the unique combination of inherited traits.
Rules for the Inheritance of Traits — Mendel's Contributions
The rules of inheritance are based on the fact that in sexual reproduction, both the father and mother contribute nearly equal amounts of genetic material to their child. This means every trait in the child is influenced by both paternal and maternal DNA. As a result, for each trait, a child inherits two versions, one from each parent.
Gregor Mendel was a scientist who worked out the fundamental rules of inheritance through his experiments with garden pea plants. He was the first to use mathematics to analyze inheritance patterns by counting the individuals with specific traits in each generation.
Mendel's Experiments with Pea Plants
Mendel studied several contrasting, visible characters in peas, such as:
- Tall vs. short plants
- Round vs. wrinkled seeds
- Violet vs. white flowers
Monohybrid Cross (Inheritance of a Single Trait)
- Parental Cross: Mendel crossed a tall pea plant with a short pea plant.
- First Generation (F1): All the offspring plants in the F1 generation were tall. There were no "medium-height" plants, meaning one trait completely masked the other.
- Second Generation (F2): Mendel then allowed the F1 tall plants to self-pollinate. In the resulting F2 generation, not all plants were tall. Instead, about one-quarter of them were short.
Mendel's Conclusions:
- This result showed that the F1 plants had inherited both the "tallness" and "shortness" traits from the original parents, but only the tallness trait was expressed.
- Mendel proposed that traits are controlled by two copies of factors (which we now call genes). These can be identical or different.
- Dominant Trait: A trait like tallness ('T'), which expresses itself even when only one copy is present, is called a dominant trait. Both TT and Tt plants are tall.
- Recessive Trait: A trait like shortness ('t'), which only expresses itself when two copies are present, is called a recessive trait. Only tt plants are short.
Dihybrid Cross (Inheritance of Two Traits)
Mendel also studied the inheritance of two different traits at the same time, for example, plant height and seed shape.
- Parental Cross: He crossed a tall plant with round seeds with a short plant with wrinkled seeds.
- First Generation (F1): All the offspring were tall and had round seeds. This confirmed that tallness and round seeds are dominant traits.
- Second Generation (F2): When the F1 plants self-pollinated, the F2 generation showed four different combinations:
- Tall plants with round seeds (parental type)
- Short plants with wrinkled seeds (parental type)
- Tall plants with wrinkled seeds (new combination)
- Short plants with round seeds (new combination)
Conclusion on Independent Inheritance: The appearance of new combinations in the F2 generation showed that the trait for height (tall/short) and the trait for seed shape (round/wrinkled) are inherited independently of each other.
How do these Traits get Expressed?
The mechanism of heredity is rooted in the cell's genetic material.
- DNA and Proteins: Cellular DNA is the information source for making proteins in a cell.
- Gene: A gene is a specific section of DNA that contains the instructions for making one protein.
- Genes Control Traits: These proteins then control the characteristics, or traits, of an organism.
- Plants have hormones that trigger growth. The amount of hormone determines the plant's height.
- An enzyme is required to produce this hormone.
- The gene for this enzyme controls how efficiently it works.
- If the gene directs the enzyme to work efficiently (the dominant trait), a lot of hormone is made, and the plant will be tall.
- If the gene has an alteration that makes the enzyme less efficient (the recessive trait), less hormone is produced, and the plant will be short.
The Mechanism of Inheritance
For traits to be inherited from both parents, the following mechanism is used in sexually reproducing organisms:
- Each individual has two sets of all genes, one set inherited from each parent.
- These genes are located on structures called chromosomes. Genes are not on one single long thread of DNA but on separate chromosomes.
- Each body cell has two copies of each chromosome—one from the male parent and one from the female parent.
- To ensure the offspring gets the correct number of chromosomes, the germ-cells (sperm and egg) must have only one copy of each chromosome.
- When two germ cells combine during fertilization, the normal number of chromosomes is restored in the offspring, ensuring the stability of the species' DNA.
Sex Determination
The sex of a newborn individual is determined by different strategies across different species.
- Environmental Cues: In some reptiles, the temperature at which fertilized eggs are incubated determines whether the offspring will be male or female.
- Non-Genetic: In some animals like snails, individuals can change their sex, meaning it is not genetically fixed.
Sex Determination in Human Beings
In humans, sex is primarily determined genetically by specific chromosomes called sex chromosomes.
- Most human chromosomes exist in matching pairs. We have 22 such pairs.
- The 23rd pair is the sex chromosome pair, which is not always a perfect match.
- Females have a perfect pair of sex chromosomes, both called X. A female's genetic makeup is XX.
- Males have a mismatched pair, with one normal-sized X chromosome and one shorter Y chromosome. A male's genetic makeup is XY.
Inheritance of Sex Chromosomes:
- A mother has two X chromosomes (XX), so all her eggs will contain a single X chromosome.
- A father has one X and one Y chromosome (XY), so half of his sperm will carry an X chromosome, and the other half will carry a Y chromosome.
The sex of the child is therefore determined by which sperm fertilizes the egg:
- If a sperm carrying an X chromosome fertilizes the egg, the child will have an XX combination and will be a girl.
- If a sperm carrying a Y chromosome fertilizes the egg, the child will have an XY combination and will be a boy.