Introduction to Genetics

• Genetics - the scientific study of heredity.

What is Genetics?

• Genetics scientifically studies heredity.
• Heredity is the inheritance of biological characteristics.
• Genetics studies gene structure and action, as well as inheritance patterns from parent to offspring.
• It deals with the inheritance of biological characteristics.
• Genetics is used in understanding: evolution, development, ecology, and molecular biology, and variation in human health and disease.

Gregor Mendel

• Gregor Mendel was a 19th-century scientist who published his genetics ideas in 1866.
• Mendel's work was largely unrecognized until 1900, long after his death.
• Mendel acquired his understanding of genetics mostly through pea plant breeding experiments.
• Mendel was born in 1822 in Czechoslovakia.
• In 1843, Mendel became a monk at a monastery.
• Mendel taught biology and had interests in statistics.
• Mendel also studied at the University of Vienna.

Genes and Alleles

• Genes are units of inheritance, occurring at specific locations (loci) on a chromosome.
• Genes are responsible for hereditary characteristics in plants and animals.
• Alleles are alternate forms of the same gene, which may result in different expressions of a trait.
• A dominant allele masks the presence of another allele in the phenotype.
• A recessive allele is masked in the phenotype by the presence of another allele.

Genotype and Phenotype

• A genotype is the genetic makeup of an individual for a trait.
• A phenotype is the physical appearance of an organism.
• A homozygous organism has two identical alleles (e.g., YY or yy).
• A heterozygous organism has two different alleles (e.g., Yy).

DNA and Inheritance

• DNA is the genetic material.
• Nucleic acids store and transmit hereditary information.
• A gene is a unit of inheritance.
• Genes are made of DNA, specifically deoxyribonucleic acid.

Chromosomes

• Eukaryotic chromosomes have linear DNA molecules associated with many proteins.
• Chromatin, a complex of DNA and protein, is found in the nucleus of eukaryotic cells.
• Chromosomes fit into the nucleus through an elaborate, multilevel packing system.

Heredity and Variation

• Genetics is the scientific study of heredity and variation.
• Heredity is the transmission of traits from one generation to the next.
• Variation is differences in appearance that offspring show from parents and siblings.
• Genes are units of heredity made of DNA segments.
• Genes pass to the next generation through reproductive cells called gametes (sperm and eggs).
• A gene has a specific location called a locus on a certain chromosome.
• Most DNA is packaged into chromosomes.
• One set of chromosomes is inherited from each parent.

Asexual vs Sexual Reproduction

• In asexual reproduction, one parent produces genetically identical offspring by mitosis.
• A clone is a group of genetically identical individuals from the same parent.
• In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents.

Chromosome Sets in Human Cells

• Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes.
• A karyotype is an ordered display of chromosome pairs from a cell.
• The two chromosomes in each pair are homologous chromosomes (homologs).
• Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters.
• Sex chromosomes are X and Y; females have XX, and males have XY.
• The 22 chromosome pairs not determining sex are autosomes.
• Each homologous chromosome pair includes one chromosome from each parent.
• The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father.
• A diploid cell (2n) has two chromosome sets; for humans, 2n = 46.

Meiosis

• A gamete (sperm or egg) contains a single set of chromosomes and is haploid (n).
• For humans, the haploid number is 23 (n = 23).
• An unfertilized egg (ovum) has an X chromosome; a sperm cell has either X or Y.
• At sexual maturity, ovaries and testes produce haploid gametes by meiosis, not mitosis.
• Meiosis results in one chromosome set in each gamete.
• Fertilization is the union of gametes (sperm and egg).
• A fertilized egg is a zygote and has one chromosome set from each parent.
• A zygote produces somatic cells by mitosis to become an adult.
• A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism.

Meiosis Reduces Chromosome Number

• Like mitosis, meiosis is preceded by chromosome replication.
• Meiosis takes place in two sets of cell divisions: meiosis I and meiosis II.
• The two divisions result in four daughter cells, rather than two in mitosis.
• Each daughter cell has half as many chromosomes as the parent cell.
• In meiosis I, homologous chromosomes separate, resulting in two haploid daughter cells with replicated chromosomes; it is called the reductional division.
• In meiosis II, sister chromatids separate--results in four haploid daughter cells.
• Meiosis II is called the equational division.
• Meiosis I is preceded by interphase, in which chromosomes replicate to form sister chromatids, joined at the centromere.
• The single centrosome replicates, forming two centrosomes.
• Division in meiosis I occurs in four phases.

Stages of Meiosis I

• In prophase I, homologous chromosomes loosely pair up, aligned gene by gene forming a tetrad, a group of four chromatids.
• Nonsister chromatids exchange DNA segments.
• Each tetrad has one or more chiasmata, X-shaped regions crossing over occurred.
• In metaphase I, tetrads line up at the metaphase plate, with one chromosome of each tetrad facing each pole.
• Microtubules from one pole attach to the kinetochore of one chromosome of each tetrad, while microtubules from the other pole attach to the kinetochore of the other chromosome.
• In anaphase I, pairs of homologous chromosomes separate, one chromosome moving toward each pole, guided by the spindle apparatus.
• Sister chromatids remain attached at the centromere and move as one unit toward the pole.
• In telophase I, each half of the cell has a haploid chromosome set; each chromosome still consists of two sister chromatids.
• Cytokinesis usually occurs simultaneously, forming two haploid daughter cells.
• No chromosome replication occurs between the end of meiosis and the beginning of meiosis II because the chromosomes are already replicated.

Stages of Meiosis II

• Division in meiosis II also occurs in four phases, like mitosis.
• In prophase II, a spindle apparatus forms; in late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate.
• In metaphase II, the sister chromatids are arranged at the metaphase plate.
• Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical.
• Kinetochores of sister chromatids attach to microtubules extending from opposite poles.
• In anaphase II, the sister chromatids separate, and forming chromosomes move as two newly individual chromosomes toward opposite poles.
• In telophase II and cytokinesis, chromosomes arrive at opposite poles, and nuclei form, while the chromosomes begin decondensing.
• Cytokinesis separates the cytoplasm.
• At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes.
• Each daughter cell is genetically distinct from the others and from the parent cell.

Independent Assortment of Chromosomes

• Homologous chromosome pairs orient randomly at metaphase I of meiosis.
• Independent assortment each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs.
• When chromosomes assort independently into gametes, the number of combinations possible is 2", where s the haploid number.
• Natural selection results in the accumulation of genetic variations favored by the environment.

Genetic Variation and Sexual Reproduction

• Sexual reproduction contributes to the genetic variation in a population, which originates from mutations.
• Mutations (changes in an organism's DNA) are the original source of genetic diversity.
• Mutations create different versions of genes called alleles.
• Reshuffling of alleles during sexual reproduction produces genetic variation.
• The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation.
• There are three mechanisms contribute to genetic variation: Independent assortment of chromosomes, crossing over and random fertilization.

Recombinant Chromosomes

• Crossing over produces recombinant chromosomes, which combine genes inherited from each parent.
• Crossing over begins very early in prophase I, when homologous chromosomes pair up gene by gene.
• Homologous portions of two nonsister chromatids trade places.
• Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome.

Random Fertilization

• Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg).
• The fusion of two gametes—each with 8.4 million possible chromosome combinations from independent assortment —produces a zygote with about 70 trillion diploid combinations.
• Crossing over adds even more variation.
• Each zygote has a unique genetic identity.

Synapsis and Chromosome Behavior

• Three events are unique to meiosis and occur in meiosis 1:
• Synapsis and crossing over in prophase 1: Homologous chromosomes physically connect and exchange genetic information.
• At the metaphase plate, there are paired homologous chromosomes (tetrads) instead of individual replicated chromosomes.
• At anaphase I, it is homologous chromosomes instead of sister chromatids that separate.
• Sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I.
• Protein complexes called cohesins are responsible for this cohesion.
• In mitosis, cohesins are cleaved at the end of metaphase.
• In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids)