Genetics: Heredity, Genes and Chromosomes

Learning Objectives

• Genetics definition and the relationship between genes and chromosomes
• Identify and define standard genetic terminology
• Molecular basis of inheritance patterns
• Recognize and work with autosomal and sex-linked inheritance patterns
• Track inheritance across generations using human pedigrees

Genetics

• Study of heredity and variation.
• Living organisms reproduce their own kind.
• Heredity is the transmission of traits from one generation to the next.
• Variation is demonstrated by differences in appearance that offspring show from parents and siblings.

Genes

• The units of heredity.
• Specific sequences of DNA.
• Each gene has a specific chromosomal location referred to as the locus.
• Alleles are variations in a gene.

Chromosomes

• All DNA in a cell is the genome.
• A genome is a single DNA molecule in prokaryotic cells, or several DNA molecules called chromosomes in eukaryotes.
• Each chromosome contains one long DNA molecule.
• Eukaryotic species all have a characteristic number of chromosome.
• Somatic cells (non-reproductive cells) in humans have two sets of chromosomes, totaling 46.
• Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as the somatic cells.
• A zygote is created through the fusion of gametes, and it has two full sets of chromosomes.
• DNA is replicated and the chromosome condenses in preparation for cell division.
• A duplicated chromosome consists of two sister chromatids which are joined copies of the original chromosome.

Division

• The centromere is the constriction seen in the duplicated chromosome, where sister chromatids are most closely attached.
• Sister chromatids separate during cell division and move into two nuclei.
• Once separated, the former sister chromatids are called chromosomes.
• Chromosomes exist in pairs with homologous chromosomes, which have same length, centromere position, and genes.
• One of each chromosome pair comes from each parent; humans have 23 pairs.
• A karyotype is an image of chromosome pairs arranged by size/shape.

Describing Chromosomes

• Diploid cells have two complete sets of chromosomes.
• One set is inherited from each parent.
• Human life cycle begins when a haploid sperm fuses with haploid cells resulting in a diploid zygote which is fertilization
• Offspring are genetically variable from their parents.
• Gametes originate from germ cells which are specialized and found in the gonads.
• Meiosis, which is modified cell division, produces gametes that consist of two rounds of division but only a single round of DNA replication.

Genotype and Phenotype

• Geneticists describe organisms based on their genotype and phenotype.
• Genotype refers to the genetic makeup.
• Phenotype is the organism's physical appearance
• When describing a genotype, it involves describing the combination of alleles at a locus.
• Homozygous means both alleles are the same at a locus.
• Heterozygous means both alleles are different at a locus.
• Dominant affects the appearance of the organism if present.
• Recessive allele has no noticeable effect on phenotype if a dominant allele is present.

Genetic Crosses

• It describes individuals based on generation and breeding capabilities.
• P generation refers to the first generation of cross.
• F₁ generation is the offspring of the P generation.
• F₂ generation results from self-fertilizing or interbreeding F₁ plants.
• True-breeding means homozygous individuals that produce offspring matching the genotype and phenotype of the parents.
• Hybrids are offspring from a cross involving two true-breeding parents.
• Classify individual phenotypes based on how common they are in nature.
• Wild-type is the most common phenotype.
• Mutations are variations on the wild-type.

Punnett Squares

• Used to predict the outcomes of a genetic cross.
• The Punnett square shows gametes, haploid, from both parents and the resulting zygotes, diploid.

Mendel's Law

• Used to determine gametes.
• The law of segregation relates to movement of chromosomes during meiosis.
• Monohybrid cross involves breeding two individuals that are both heterozygous at a single locus.
• Dihybrid cross involves breeding two individuals that are both heterozygous at two separate loci.
• When determining gametes, include one allele from each locus.
• Describe outcomes of genetic crosses by presenting genotypic and phenotypic ratios.
• Helps predict the probability of a specific genotype/phenotype occurring from a cross.

Model Organisms

• Model organisms are species easily studied in the lab, and the results of which can be applied to larger groups.
• Size, lifespan, care requirements, and ethical concerns determine the model organism.

Molecular Level of Alleles

• Structural differences in alleles lead to functional differences at the protein level.
• Alleles are variations in a gene (or locus).
• Products of variation in the DNA sequence at a specific locus.
• These variations can range from one to thousands of nucleotides.
• Arise due to mutations (from replication errors, DNA damage).

Genes

• Genes often encode for proteins with specific functions.
• The central dogma explains that DNA encodes RNA encodes protein.
• Each triplet codon, consisting of a three-letter nucleotide sequence, encodes for a specific amino acid in the protein.
• Mutations in the DNA sequence can change the amino acid sequence in a protein.
• Varying outcomes on protein function can result from the mutation.
• A null allele comes from a mutant allele to cause a completely non-functional protein.
• A leaky alleles result in reduced protein function.
• Silent mutations do not affect the amino acid sequence.

Dominance

• Defined as the phenotype shown by the heterozygote.
• Describes the phenotype; therefore, describes alleles.
• Recessive phenotypes appear only in the absence of a dominant allele.
• Observed when null mutations in genes are functionally haplosufficient; the gene produces enough protein to exert a wild-type affect.
• Haplosufficient - When one functional copy of a gene has enough function to produce a wild-type phenotype.
• The dominant allele produces the functional protein in a heterozygote, while the recessive allele produces a non-functional protein.
• Some genes are haploinsufficient as one functioning copy is not enough to produce a wild-type function.
• A null mutation becomes dominant if the genes are haploinsufficient since the heterozygote can't produce normal function.
• Achondroplasia caused by a mutation results in a non-functional fibroblast growth factor.
• Some mutations produce a new function for that gene.

Concept Check

• A gain-of-function mutation is almost always dominant (acts like a different gene).
• Wild-type allele does not mask the new function.

Explore genetics, focusing on heredity, genes, and chromosomes. Learn about the molecular basis of inheritance patterns, autosomal and sex-linked inheritance, and tracking inheritance using pedigrees. Understand the relationship between genes and chromosomes.