Genetics is the study of how traits are inherited from one generation to the next. Traits are variations in physical characteristics determined by proteins, which in term are determined by genes- segments of DNA that direct protein production. In eukaryotes, chromosomes carry genes, and each cell typically contains two copies of each nuclear chromosome, one from each parent. Organelles like mitochondria and chloroplasts have they own DNA, which is typically inherited from the maternal parent only. During meiosis, gametes (eggs and sperm) are produced, each containing one copy of each chromosome. When fertilization occurs, the offspring receive two copies of each gene. These copies may differ and are known as alleles. The genetic makeup of an individual is called their genotype, which influences their observable traits, or phenotype.
15.2 Genes, inheritance and meiosis
Elizabeth Dahlhoff and Melissa Hardy
Learning Objectives
By the end of this section, you will be able to do the following:
- Understand the central importance of DNA as information molecule for genetics.
- Be able to differentiate between genotype and phenotype.
- Recognize the relationship between alleles, allelic variation, and homologous chromosomes.
- Review role of meiosis in genetic information flow.
Molecular basis of genetics and evolution
Figure 2: Nuclear versus organelle (maternal) inheritance (Image credit: Understanding Science Website, UC Berkeley).
Meiosis
Meiosis is the process that produces eggs and sperm. Eggs and sperm are also known as gametes. During meiosis, one copy of each paired chromosome is moved into the gamete. Cells with one copy of each chromosome are known as “haploid“. This separation, or segregation, of the homologous (paired) chromosomes means also that only one of the copies of the gene gets moved into a gamete. The offspring are formed when that gamete unites with one from another parent and the two copies of each gene (and chromosome) are restored.
The offspring will receive two copies of each nuclear gene (one from each parent), but the copies are not necessarily identical. You already knew this – you don’t get identical information from your mother and your father because they have different DNA (which gives them different traits). The different versions of one specific gene are known as alleles. As you learn about genetics, you will learn about how the information from both alleles of a specific gene interact to give an individual their trait. The genetic information that an individual has is called their genotype. The genotype of an individual produces the individual’s phenotype, or physical traits.
Figure 3. Meiosis. During meiosis, the DNA is copied once, then the cell divides twice. This produces cells with half as much genetic information as the original cell (1 copy of each chromosome). These cells become the sex cells (eggs or sperm). When two sex cells unite during fertilization, the original number of chromosomes (2 copies of each one) is restored (Photo credit: Wikimedia Commons).
Watch this great Amoeba Sisters video for a review of meiosis:
Section Summary
Glossary
- allele
- Different versions of a gene that can exist at the same locus (position) on homologous chromosomes. Alleles can lead to different traits or characteristics.
- diploid
- A cell that contains two sets of chromosomes, one from each parent. Most cells in organisms, including humans, are diploid.
- fertilization
- The process in which a sperm cell and an egg cell join, combining their genetic material to form a zygote, which then develops into an offspring.
- genotype
- The genetic makeup of an individual, consisting of the alleles inherited from both parents. It determines the potential traits an individual can exhibit.
- haploid
- A cell that contains one complete set of chromosomes (half the number found in diploid cells). Gametes (sperm and egg cells) are haploid.
- homologous
- Chromosomes that are similar in size, shape, and genetic content, one inherited from each parent. They carry the same genes, though they may have different versions (alleles).
- meiosis
- A type of cell division that reduces the chromosome number by half, producing gametes (sperm and egg cells). It ensures genetic diversity and allows for sexual reproduction.
- phenotype
- The observable physical or biochemical characteristics of an individual, resulting from the interaction of their genotype and environmental factors.
References
Media Attributions
- Overview-of-the-central-dogma-shows-the-flow-of-genetic-information-inside-a-biological
- Amoeba sisters- deconstructed DNA
- 66390_evo_resources_resource_image_175_original
- Meiosis_Overview_new.svg
