Heredity

Heredity is the process by which traits and characteristics are passed from parents to offspring through genes. Genes are segments of DNA that carry instructions for protein synthesis, influencing traits like height and eye color. For example, a child may inherit the tallness trait from their parents due to specific gene combinations.

Mendel crossed pea plants with contrasting traits, such as tall and short plants, to study inheritance. He found that traits like tallness were dominant, appearing in the first generation, while shortness reappeared in the second generation. This demonstrated the principles of dominance and segregation in genetics.

Dominant traits are expressed when at least one dominant allele is present, masking recessive traits. Recessive traits only appear when two recessive alleles are inherited. For example, in pea plants, the tall trait (T) is dominant over the short trait (t), so Tt plants are tall.

Variations arise from mutations, genetic recombination during sexual reproduction, and environmental factors. Sexual reproduction maximizes variation by combining genes from two parents. For instance, siblings may look different due to unique gene combinations inherited from their parents.

Variations provide adaptability, allowing species to survive changing environments. Beneficial traits, like heat resistance in bacteria, increase survival chances. Over time, these traits become common through natural selection, ensuring species continuity.

Mendel's law states that genes for different traits are inherited independently. For example, seed color and shape in peas are inherited separately, leading to new trait combinations in offspring. This principle explains genetic diversity in sexually reproducing organisms.

Sex is determined by sex chromosomes: females have XX, and males have XY. The father contributes either an X or Y chromosome, while the mother always contributes an X. An X from the father results in a girl (XX), and a Y results in a boy (XY).

Genes are DNA segments that provide instructions for protein synthesis. Proteins influence traits, like enzymes for hormone production affecting plant height. For instance, an efficient enzyme gene leads to more growth hormone, resulting in a tall plant.

Alleles are different versions of a gene. For example, the gene for flower color in peas has alleles for violet (V) and white (v). A plant with Vv will have violet flowers because V is dominant, while vv plants will have white flowers.

Asexual reproduction produces minimal variation due to identical DNA copying, leading to similar offspring. Sexual reproduction creates significant variation by combining genes from two parents. For example, sugarcane plants show little variation, while humans exhibit diverse traits.

A monohybrid cross involves one trait, like seed color in peas. Mendel's cross between yellow (YY) and green (yy) seeds produced all yellow F1 offspring (Yy), showing dominance. The F2 generation had a 3:1 ratio of yellow to green seeds, illustrating segregation.

A dihybrid cross involves two traits, like seed color and shape in peas. Mendel's cross between round yellow (RRYY) and wrinkled green (rryy) seeds produced F1 offspring with round yellow seeds (RrYy). The F2 generation showed a 9:3:3:1 ratio, demonstrating independent assortment.

Environmental factors can affect gene expression, leading to phenotypic variations. For example, temperature can determine sex in some reptiles, and nutrition can influence human height. However, these changes are not inherited unless they alter DNA.

Chromosomes carry genes and ensure their accurate distribution during cell division. Humans have 23 pairs, with one set from each parent. During reproduction, gametes carry half the chromosomes, restoring the full set in offspring to maintain genetic stability.

Recessive traits can skip generations if masked by dominant alleles in parents. For example, a child may show a recessive trait (aa) if both parents carry the recessive allele (Aa). The trait reappears when two recessive alleles combine in offspring.

Perform a test cross by breeding the tall plant with a homozygous recessive (tt) short plant. If all offspring are tall, the plant is homozygous dominant (TT). If some are short, it is heterozygous (Tt), indicating it carries the recessive allele.

The cross Tt x Tt yields a 3:1 phenotypic ratio of tall to short plants. The genotypic ratio is 1 TT : 2 Tt : 1 tt. Thus, there is a 75% chance of tall offspring (TT or Tt) and a 25% chance of short offspring (tt).

Blood groups are determined by multiple alleles (IA, IB, i). IA and IB are codominant, while i is recessive. For example, IAi results in blood group A, IBIB in B, IAIB in AB, and ii in O. This system shows multiple allele inheritance and codominance.

Genotype refers to an organism's genetic makeup, like TT or Tt for tallness. Phenotype is the physical expression of traits, such as being tall or short. For example, TT and Tt genotypes both result in a tall phenotype, while tt results in short.

Mendel's principles explain how traits are inherited and predict offspring traits. They form the foundation of modern genetics, aiding in breeding programs and genetic research. For example, farmers use these principles to develop crops with desired traits.

Use the mnemonic 'Dominant is Demanding'—dominant traits always show up when present. Recessive traits are 'shy' and only appear when two recessive alleles are inherited. For example, in peas, tall (T) is dominant over short (t), so Tt plants are tall.

Studying heredity helps understand genetic disorders, improve crop yields, and develop medical treatments. For example, knowing inheritance patterns can predict disease risks in families. It also explains biodiversity and evolutionary processes.

Mutations are DNA changes that can create new traits. If they occur in gametes, they can be inherited. For example, a mutation in a flower color gene may produce a new color variant, which can be passed to offspring if the mutation is in reproductive cells.

DNA carries genetic information passed from parents to offspring. It codes for proteins that determine traits. For example, DNA sequences dictate eye color by instructing pigment production. Accurate DNA replication ensures genetic continuity across generations.

Mendel's laws help predict traits in offspring, useful in agriculture and medicine. For example, breeders use them to develop disease-resistant crops. Genetic counselors use them to assess inheritance risks for conditions like sickle cell anemia.