Heredity and Inheritance: Understanding Genetics
Heredity and Inheritance
What is Heredity?
Heredity is the passing of physical characteristics from parent to offspring. These characteristics are called inherited traits and are determined by one or many genes (e.g., eye color, hair and skin color, height).
Chromosome Theory of Inheritance
According to the chromosome theory of inheritance, genes are carried from parents to their offspring on chromosomes. In sexual reproduction, genetic material from two parents combines to produce a new organism, which is different from both parents.
Diploid vs. Haploid
A fertilized egg is diploid, which means it has two sets or pairs of chromosomes, one from each parent. Gametes (mature sex cells, such as an egg or a sperm) are haploid, meaning they only have one set of chromosomes. Human body cells have 46 chromosomes (23 from each parent).
Genes and Alleles
Chromosomes are made up of many genes joined together like beads on a string. Your body cells each contain between 20,000 and 25,000 genes. Each gene controls a trait.
- Gene: A segment of DNA (on a specific site on a specific chromosome) that controls the physical and inheritable traits or phenotype of an organism (e.g., eye color).
- Alleles: Different forms of a gene (e.g., blue or brown eyes). An organism’s traits are controlled by the alleles they inherit from their parents (one from each parent).
Types of Alleles
- Dominant Allele: The trait always shows.
- Recessive Allele: The trait is hidden whenever a dominant allele is present.
Phenotype and Genotype
- Phenotype: An organism’s physical appearance or visible traits.
- Genotype: An organism’s genetic makeup or allele combinations.
Homozygous vs. Heterozygous
- Homozygous: “Purebred” – has two identical alleles for a trait (both dominant or both recessive).
- Heterozygous: “Hybrid” – has two different alleles for a trait (one dominant and one recessive).
Mendel’s Experiments
Gregor Mendel was an Austrian monk who experimented in the 1850s with thousands of pea plants, looking at their different traits to understand the process of heredity. His discoveries gave him the informal title of “father of genetics”.
Mendel’s Findings
- Mendel found mathematical patterns of inheritance.
- Changed previous views of inheritance (blending, spermists, ovists).
Mendel’s Crosses
Firstly, Mendel cross-pollinated plants with contrasting traits: purebred (homozygous) tall (TT) with purebred (homozygous) short (tt) – Parental (P) generation. Offspring from the first cross were called F1 (filial), and they were all Tt hybrids. Then, when the F1 plants were fully grown, Mendel allowed them to self-pollinate. The F2 generation was a mix of tall and short: ¾ were tall (TT or Tt) and ¼ were short (tt) (heterozygous).
Mendel’s Conclusions
- Traits exist in pairs (e.g., tall and short).
- Each parent contributes one factor.
- There were no plants of medium height (no blending).
- One factor in a pair can be dominant and mask or hide the other factor (recessive).
We now refer to “factors” as genes.
Chromosomes and Genes
Notice that each chromosome in this example has the same genes. The genes are lined up in the same order on both chromosomes. The alleles for some of the genes might be different. For example, the organism has the A allele on one chromosome and the a allele on the other (heterozygous). This organism is heterozygous for some traits and homozygous for others.
Codominance
In codominance, the alleles are neither dominant nor recessive, so both alleles are expressed in the offspring. For example, a white chicken crossed with a black chicken has offspring that are mixed black and white.
Probability and Punnett Squares
Probability is the number that describes how likely it is that a certain event will occur. Probability can be expressed as a ratio, fraction, or percent (1:2, ½, 50%). For example, the chance of tossing heads in a coin toss is 1 out of 2, or 1:2, ½, or 50%. The principles of probability can be used to predict the results of genetic crosses.
A Punnett Square is a chart that shows all the possible combinations of alleles that can result from a genetic cross.
Asexual Reproduction
Asexual reproduction is a type of reproduction by which offspring arise from a single parent and inherit the genes of that parent only (cloning). The original sex cell divides twice, producing four sex cells that have ½ the number of chromosomes! When sex cells combine to form a new organism, each sex cell contributes half the normal number of chromosomes. Thus, the offspring gets the normal number of chromosomes – half from each parent.
Cell Division: Mitosis and Meiosis
Cell division and reproduction can occur in two ways: mitosis and meiosis.
- Mitosis: A process of cell duplication, or reproduction, during which one cell gives rise to two genetically identical daughter cells. It’s used by single-celled organisms to reproduce; it is also used for the organic growth of tissues, fibers, and membranes.
- Meiosis: A division of a parent cell, destined to become a sex cell, involving a double split of the nucleus and giving rise to four gametes, or sex cells, each possessing half the number of chromosomes of the original parent cell. It’s useful for sexual reproduction of organisms. The male and female sex cells, e.g., the sperm and egg, fuse to create a new, singular biological organism.