BIOL 224 Single Gene Inheritance Lecture Slides PDF

Lecture 01 Single gene inheritance BIOL 224 Jan. 13th, 2025 1 Learning objectives 1. Define genetics and understand the relationship between genes and chromosomes. 2. Identify and define standard genetic terminology. 3. Understand the molecular basis of...

Lecture 01 Single gene inheritance BIOL 224 Jan. 13th, 2025 1 Learning objectives 1. Define genetics and understand the relationship between genes and chromosomes. 2. Identify and define standard genetic terminology. 3. Understand the molecular basis of inheritance patterns. 4. Be able to recognize and work with both autosomal and sex-linked inheritance patterns. 5. Use human pedigrees to track inheritance across generations 2  Genetics is the Key Concept study of heredity and variation 3 Offspring acquire genes from parents by inheriting chromosomes Living organisms are distinguished by their ability to reproduce their own kind Genetics is the scientific study of heredity and variation 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 4 Genes are stored in DNA packaged into chromosomes Genes = units of heredity Specific sequence of DNA Each gene has a specific chromosomal location = locus (plural, loci) Variations in a gene are called alleles 5 The large, complex chromosomes of eukaryotes duplicate with each cell division All the DNA in cell constitutes its genome A genome can consist of a single DNA molecule (common in prokaryotic cells), or several DNA molecules (common in eukaryotic cells) = chromosomes Each chromosome contains one long DNA molecule 6 The large, complex chromosomes of eukaryotes duplicate with each cell division Every eukaryotic species has a characteristic number of chromosomes In humans, somatic cells (non-reproductive cells) have two sets of chromosomes (2 × 23 = 46 total) Gametes (reproductive cells: sperm and eggs) have half as many chromosomes (i.e., only one set) as somatic cells Fuse to create a zygote with two full sets of chromosomes 7 The large, complex chromosomes of eukaryotes duplicate with each cell division In preparation for cell division, DNA is replicated and chromosomes condense Each duplicated chromosome consists of two sister chromatids (joined copies of the original chromosome) 8 The large, complex chromosomes of eukaryotes duplicate with each cell division The centromere is the constriction that can be seen in the duplicated chromosome, where sister chromatids are most closely attached 9 The large, complex chromosomes of eukaryotes duplicate with each cell division During cell division, sister chromatids separate and move into two nuclei Once they have been separated, the sister chromatids are called chromosomes 10 Genes are contained within chromosomes Chromosomes can exist in pairs with same length, centromere position, and genes = homologous chromosomes One of each pair inherited from each parent 23 pairs in humans Autosomes (22 pairs in humans) and sex chromosomes (X/Y in humans) Represented in a karyotype = image of chromosome pairs arranged by size/shape 11 Describing chromosomes Cells with two complete sets of chromosomes are diploid (2n) One set inherited from each parent 12 Gametes have a single set of chromosomes Gametes (eggs and sperm) are haploid (n) cells with a single set of chromosomes Human life cycle begins when haploid sperm fuses with haploid egg to create diploid zygote = fertilization Results in offspring that *genetically variable* to parents 13 Behaviour of chromosome sets in the human life cycle Gametes originate from germ cells = specialized cells found in the gonads (ovaries, testes) Produced by meiosis = modified cell division consisting of two rounds of division but only a single round of DNA replication Diploid (2n) → haploid (n) 14 Concept check An organism has a ploidy of 4n = 16. How many different types of chromosomes does this organism have? TRUE or FALSE: A pair of connected sister chromatids represents two separate chromosomes. 1. TRUE 2. FALSE 15  Geneticists describe Key Concept organisms based on their genotype and phenotype 16 The relationship between genotype and phenotype When performing genetic analyses, we are often interested in both the genetic basis of trait as well as its physical expression Genotype = genetic makeup Phenotype = physical appearance 17 The relationship between genotype and phenotype When describing the genotype, we describe the combination of alleles at a locus Homozygous = both alleles are the same a locus Heterozygous = both alleles are different at a locus 18 The relationship between genotype and phenotype We can classify alleles based on their affect on the phenotype Dominant = if present, will affect appearance of organism Recessive = has no noticeable affect on phenotype if dominant allele present 19 Genetic crosses When performing genetic crosses, we describe individuals based on generation and breeding capabilities P generation = first generation of crosses F1 generation (F for filial, Latin for “son”) = offspring of P generation F2 generation = offspring produced by self- fertilizing or interbreeding F1 plants 20 Genetic crosses True-breeding = homozygous individuals that also produce offspring that match the genotype and phenotype of parents Hybrids = resulting offspring from a cross involving two true-breeding parents 21 Genetic crosses Can also describe individual phenotypes based on their prevalence in nature Wild-type = most common phenotype in nature Mutant = variations on wild-type 22  Gametes from parent #1 Punnett squares When predicting the outcomes of a genetic cross, often will use a Punnett square Shows gametes (haploid) from both parents as well as resulting zygotes (diploid) Gametes from parent #2 Potential zygotes 23 Determining gametes To determine gametes, use Mendel’s law of segregation Due to movement of chromosomes during meiosis 24 Types of genetic crosses Monohybrid cross = breeding two individuals that are both heterozygous at a single locus Dihybrid cross = breeding two individuals that are both heterozygous at two separate loci When determining gametes, you need to include one allele from each locus! 25 Describing outcomes of genetic crosses We describe the outcomes of genetic crosses by presenting genotypic and phenotypic ratios Helps us predict the probability of a specific genotype/phenotype occurring from a cross 26 Model organisms Not all organisms are easily studied Size Lifespan Care requirements Ethical concerns Model organism = a species that can be readily studied in lab, the results of which can be applied to larger groups 27 Concept check An organism contain two identical copies of an allele is said to be… 1. Homozygous 2. Heterozygous 3. Hemizygous In a Punnett square, what do the letters inside the box represent? What about outside the box? 28  Structural differences in Key Concept alleles results functional differences at the protein level 29 Structural differences between alleles at the molecular level Alleles = variations of a gene (or locus) Products of variation of the DNA sequence at a particular locus One to 100s or 1000s of nucleotides Arise due to mutations (from replication errors, DNA damage) 30 Relationship between genotype and phenotype Genes often encode for proteins with specific functions Central dogma = DNA encodes RNA encodes protein Each triplet codon (3 letter nucleotide sequence) encodes for a specific amino acid in the protein 31 Functional differences of alleles at the molecular level Mutations in the DNA sequence can alter the sequence of amino acids in a protein Varying outcomes on function A mutant allele that results in a completely non-functional protein is called a null allele A mutant allele that results in reduce protein function is called a leaky allele Some mutations do not affect amino acid sequence = silent mutations 32 Dominance and recessiveness Dominance is defined as the phenotype shown by the heterozygote Really describes the phenotype, but we often use it describe alleles Recessive phenotypes only appear in the absence of a dominant allele How do we get dominant/recessive alleles? 33 Dominance and recessiveness We see dominant/recessive relationships between alleles when null mutations in genes are functionally haplosufficient When one functional copy of a gene has enough function to produce wild-type phenotype In a heterozygote, the dominant allele produces the functional protein, whereas the recessive allele produces a non-functional protein A dominant allele does not suppress the recessive allele! 34 Dominance and recessiveness 35 Dominance and recessiveness Some genes are haploinsufficient = one functional copy of a gene is not enough to produce wild-type function In this case, a null mutation is dominant, since the heterozygote can’t produce normal function E.g.) achondroplasia, mutation causes a non- functional fibroblast growth factor 36 Dominance and recessiveness Some mutations can produce a new function for that gene (gain-of- function mutation) Almost always dominant, as wild-type allele usually cannot mask new function (acts like a different gene) 37 Concept check A researcher is studying two different alleles of the gene encoding human lipase, an enzyme involved in the digestion of fats. He notices that one allele encodes a nonfunctional lipase enzyme; this is an example of a ____________ allele. The other allele encodes a “partially” functional lipase enzyme; however, the function of this enzyme is remarkably reduced compared to wild-type lipase. This is an example of a __________ allele. 1. null; leaky 2. leaky; null 3. recessive; dominant 4. dominant; recessive 38 Concept check If a gene is haploinsufficient, you would predict that… 1. a null mutant allele of this gene will behave in a dominant fashion. 2. a null mutant allele of this gene will behave in a recessive fashion. 3. an organism must carry two null alleles of this gene in order to demonstrate a mutant phenotype. 4. heterozygous diploids bearing a null mutant allele and a wild-type allele of this gene will be viable and demonstrate no mutant phenotype. 5. this gene cannot encode a protein. 39 Next class Continue with single gene inheritance (Chapter 2) Independent assortment of genes (Chapter 3) 40