Biology Mendelian Genetics notes

👶 Inheritance The passing down of traits Definitions of Structures Genes are sequences of nucleotides on a DNA molecule Alleles are alternative forms of the same gene found on the corresponding gene locus of homologous chromosomes Phenotypes are the expressed traits in an organism’s physical appearance of other attribute Genotypes are the genetic make-ups of an organism Mendelian Experiments Mendel experimented with pea plants as: It was possible to control mating with self and cross pollination There were many wild phenotypes There was a short turnaround time for results Inher it ance 1

The parental generation are true- breeding plants (they are homozygous dominant or homozygous recessive). The first filial generation are the offspring of the parental generation. They may have purple flowers but are actually hybrids (possessing a heterozygous genotype) The second filial generation are the offspring of the first filial generation. They are comprised of 1/4 homozygous dominant, 1/4 homozygous recessive, and 1/2 heterozygous genotypes but 3/4 purple phenotype and 1/4 white phenotype. The dominant allele is the allele that will express itself and gives the same phenotype in both the homozygous and heterozygous conditions. The recessive allele is the allele that will express itself only in the homozygous condition. It will fail to express itself in the heterozygous condition. Inher it ance 2

Mendelian Principles Alternate version of genes (alleles) cause variations in inherited characteristics among offspring For each character, every organism inherits one allele from each parent If 2 alleles are different, the dominant allele will be fully expressed; the recessive allele will have no noticeable effect on the offspring’s appearance Inher it ance 3

Mendel’s First Law: The Law of Segregation - the 2 alleles for each character separate during gamete formation The Law of Segregation In order to determine the zygosity of an organism for a particular trait, a Mendelian testcross may be conducted where an organism of unknown genotype (homozygous dominant or heterozygous) expressing the dominant phenotype is crossed with an organism of known genotype (homozygous recessive) expressing the recessive phenotype. The phenotype consists of the expressed physical traits whereas the genotype consists of the genetic make-up of the organism. Inher it ance 4

Non-Mendelian Genetics Mendelian genetics imply complete dominance, where the heterozygote and homozygote for the dominant allele are indistinguishable in phenotype. However, some species and genes express genes in a non-Mendelian character. Incomplete Dominance The first filial generation’s hybrids have an appearance that is “in between” that of its parents as no allele is dominant over the other - instead causing syncretised offspring. The adjacent example on red and white flowers mating to produce a pink flower is an example of incomplete dominance. Codominance Where codominance is displayed, in the heterozygous condition, both alleles have an equal effect on the phenotype. An example is that of calico cats Multiple Alleles Inher it ance 5

An example of a phenotype affected by one gene of multiple (> 2) alleles is that of human blood groups. A - I ᴬ I ᴬ (homozygous dominant) or I ᴬ i/I ᴬ I ᴼ (heterozygous) B - I ᴮ I ᴮ (homozygous dominant) or I ᴮ i/I ᴮ I ᴼ (heterozygous) AB - I ᴬ I ᴮ (codominant) O - ii/I ᴼ I ᴼ (homozygous recessive) The Law of Independent Assortment Inher it ance 6

💡The Law of Independent Assortment dictates that each pair of alleles segregates independently during gamete formation The dihybrid cross may be used to determine phenotypic ratios in accordance to Mendel’s second law. Inher it ance 7

Variations 💡Variations are differences in traits between individuals of the same species Traits of individuals are dependent on genetic and environmental interactions. Genetic variations are inheritable whereas variations caused by the environment are not. Discontinuous Variation is brought about by one or a few genes at most Continuous Variation is brought about by the additive effect of many genes For example, with the traits of height and blood groups: Continuous Variation Discontinuous Variation Deals with a range of phenotypes Deals with a few clear-cut phenotypes Controlled by many genes Controlled by one or a few genes Genes show an additive effect Genes do not show an additive effect Affected by environmental conditions Unaffected by environmental conditions Continuous Variation examples: Height/Weight Hand size Skin Color Inher it ance 8

Discontinuous Variation examples: Blood Type (O,A,B,AB) Gender (Male/Female) Presence of dimples /Absence of dimples Types of Inheritance Autosomal Inheritance Autosomal Dominant Only one mutated allele is needed for the disease to show Cannot skip generations Parents MUST be heterozygous Autosomal Recessive Both mutated alleles are needed for the disease to show Can skip generations due to carriers Parents MUST be heterozygous X-linked Inheritance X-Linked Dominant Only one X chromosome needed to contain mutated allele Affected father’s daughters will always be affected (since at least one X has to come from father). X-Linked Recessive All X chromosomes inherited have to contain the mutated allele More present in males (XY) than females (XX) Males cannot be carriers if Mother is affected (not carrier), all sons will be affected. Inher it ance 9

Diagrams: Inher it ance 10