Patterns of Inheritance
Learning Outcomes Explain the Laws of Segregation and of Independent Assortment and what features of meiosis they correspond to. Explain the terms gene, allele, dominant, recessive, genotype, phenotype, heterozygous, homozygous, Solve genetics problems involving monohybrid and dihybrid crosses. Explain incomplete dominance and co-dominance.
Mendelian Genetics father of genetics Proposed that characteristics were passed from generation to generation by heritable factors Gregor Mendel 1822-1884
Mendelian Genetics Mendel investigated many characteristics of garden peas
Stamen removal 1 2 Pollen transfer Parents (P) Carpel Stamens 3 Carpel matures into pod 4 Seeds from pod planted Offspring (F 1 )
Monohybrid Cross True Breeding Parents F1 generation
Monohybrid Cross True Breeding Parents F1 generation all purple
Monohybrid Cross F1 generation F2 generation 3 purple : 1 white
Mendel studied 7 traits. From his studies Mendel developed several ideas about the inheritance of traits.
Mendel s Conclusions A plant has two heritable factors for each character (e.g., flower color). When a plant makes gametes, only one heritable factor for each trait is given to each gamete. When gametes unite, the new plant will have a heritable factor from each parent. Some heritable factors dominate over others.
Mendel s Law of Segregation The two copies of a particular gene separate when an individual makes gametes. Each gamete receives only one copy.
Dihybrid Cross True Breeding Parents F1 generation all TALL with Purple flowers
Dihybrid Cross F1 generation PpTt x PpTt F2 generation 9 : 3 : 3 : 1
Mendel s Law of Independent Assortment The law of independent assortment. Alleles of different genes assort independently during gamete formation. This law applies to genes on different chromosomes.
Terminology Allele one version of the gene for a particular character many genes have multiple alleles Genotype the two specific alleles that an individual has --the genetic makeup of the individual Homozygous the two alleles are the same Heterozygous the two alleles are different Phenotype the appearance of the individual
Terminology Dominant presence of the allele will confer the phenotype SYMBOL USES CAPITAL LETTER Recessive the phenotype will only be seen when both alleles are recessive SYMBOL USES lowercase LETTER Co-dominant both alleles contribute to phenotype
How do Mendel s Laws apply to Meiosis?
DNA replication meiosis I 1 2 meiosis II 3 gametes (P) gametes (p) zygote (Pp)
or meiosis I meiosis I meiosis II meiosis II pt PT pt Pt
Mendel s Laws Law of Segregation each gamete contains only one heritable factor for each trait Homologous chromosomes separate at meiosis I Law of Independent Assortment heritable factors for one trait segregate independently of the heritable factors for other traits The orientation of each tetrad at the metaphase plate in Meiosis I is random (i.e., independent from orientation of other tetrads.)
Alleles occupy a physical position on a chromosome The Law of Independent Assortment applies to genes on different chromosomes. Genes on the same chromosome tend to be inherited together
Incomplete Dominance F 1 F 2
Incomplete Dominance LDL receptor HH Homozygous for ability to make LDL receptors LDL Genotypes Hh Heterozygous Phenotypes hh Homozygous for inability to make LDL receptors Cell Normal Mild disease Severe disease
Codominance Two alleles of a gene produce phenotypes that are both present in the heterozygote. Example: ABO blood group system has three alleles of the gene: I A, I B, and I O.
Codominance The A, B, O sugars are covalently linked to phospholipid headgroups on the RBC plasma membrane. O A B
Why do we care? Blood Group (Phenotype) Genotypes Carbohydrates Present on Red Blood Cells Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB A I A I A or I A i Carbohydrate A Anti-B B I B I B or I B i Carbohydrate B Anti-A AB I A I B Carbohydrate A and Carbohydrate B None O ii Neither Anti-A Anti-B No reaction Clumping reaction
Blood Group Effects on Health Type O compared to Type A, AB, and B individuals have a 14% reduced risk of squamous cell carcinoma and 4% reduced risk of basal cell carcinoma. a reduced risk of pancreatic cancer. Type B correlated with increased risk of ovarian cancer, Gastric cancer reported to be most common in blood group AB and least in group O.
More about these in Chapter 14 Pleiotropy one gene having many effects on phenotype. Epistasis the protein product of one gene controls the expression of a second gene.