Mendelian Genetics An Overview

copyright cmassengale Mendelian Genetics 4/20/2017 Gregor Johann Mendel Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants He found that the plants' offspring retained traits of the parents Called the “Father of Genetics" copyright cmassengale

Chromosomes and Genetics Genes are short regions of this DNA that hold the information needed to build and maintain the body Heredity- the passing of characters from parents to offspring Genetics- the branch of science that deals with heredity.

Pea plants have several advantages for genetics. Several characters in garden plants exist in TWO clearly different forms. Another advantage of peas is that Mendel had strict control over which plants mated with which. Self fertilization- Each pea plant has BOTH male & female reproductive organs. (stamens) –male reproductive organ (pistil)- female reproductive organs. Small Grows easily Matures quickly Produced many off-spring Fig. 14.1 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The Seven Characters Mendel Studied Character –heritable feature (physical features) Trait – each variant for a character (dominant or recessive)

Mendel’s initial experiments were Monohybrid crosses. Monohybrid Cross- a cross that involves one pair of contrasting traits. True-breeding- plants capable of producing offspring with only one form of a particular physical feature. True-breeding plant served as the P generation (Parent) and their hybrid offspring are the F1 generation. Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Ratios: Mendel’s Results When Mendel allowed the F1 plants to self-fertilize, the F2 generation included both purple-flowered and white-flowered plants. The white trait, absent in the F1, reappeared in the F2. Based on a large sample size, Mendel recorded 705 purple-flowered F2 plants and 224 white-flowered F2 plants from the original cross.

Mendel’s 4 Hypotheses Alternative versions of genes account for variations in inherited characters, which are now called alleles concept of alleles (P=purple, p=white) For each character an organism inherits two alleles, one from each parent. This set of alleles is called its genotype. If the two alleles at a locus differ, the dominant allele is fully expressed in the organism’s appearance (phenotype). The law of segregation - the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

Homozygous- when the two alleles of a particular gene present in an individual are the same Heterozygous-when the alleles of a particular gene present are different.

Mendel’s law of Segregation The two alleles for a character segregate (separate) when gametes are formed.

Mendel’s Law of Independent Assortment ~The alleles of different genes separate independently of one another during gamete formation.

Vocabulary Character –heritable feature Trait – each variant for a character True-breeding – plants that self-pollinate all offspring are the same variety Monohybrid cross – a cross that tracks the inheritance of a single character P generation – (parental) true-breeding F1- (first filial) offspring of P generation F2 – (second filial) offspring from F1 cross

Genetics and Prediction

Punnett Squares: Probability Probability – predict likelihood of an event or outcome Punnett square – grid for organizing genetic information Can be used to make predictions about a cross between two organsims Monohybrid Cross – cross between two parents and one trait

Rules to the Punnett Square 1. Figure out the genotypes of the parents. TT & tt Mom Dad

Rules to the Punnett Square 2. Figure out what kinds of gametes the parents can produce. TT & tt Mom Dad t T

Rules to the Punnett Square 3. Set up a Punnett Square for your mating. Mom T T t Tt Tt Dad Tt Tt

Rules to the Punnett Square Genotype- genetic make-up (the alleles an organism has.) Phenotype – Physical Features or characteristics.

Monohybrid Cross

Predictions for Two Traits Dihybrid Cross – cross between two parents and two traits Use a 4 x 4 Punnett square Sixteen possible outcomes

Autosomal traits vs. Sex-linked traits Autosomes – body chromosomes – first 22 pair Same in both male and female Sex Chromosomes – 1 pair, last pair XX – Female XY - Male

Sex Linked Traits Traits controlled by recessive genes located on sex chromosomes (normally associated with the X chromosome) Hemophilia – blood clotting enzyme is absent Queen Victoria’s family affected

Sex Linked Traits Red – Green Colorblindness – individuals cannot distinguish between these two colors

Females XRXR = normal XRXr = carrier XrXr = disease Males XRY = normal XrY = disease

Pedigree Charts Males = Females = Generations = Roman Numerals Individuals = Numbered sequentially Trait Expressed = Filled in Non Carriers = Empty Carrier (not ill) = Half filled Individual who carries a recessive allele that is not expressed

Pedigree Charts Chart which shows how a trait and the genes that control it are inherited within a family Identifies the presence or absence of particular trait in members of each generation

Today... Pedigree analysis In humans, pedigree analysis is an important tool for studying inherited diseases Pedigree analysis uses family trees and information about affected individuals to: figure out the genetic basis of a disease or trait from its inheritance pattern predict the risk of disease in future offspring in a family (genetic counseling)

Today... Pedigree analysis How to read pedigrees Basic patterns of inheritance autosomal, recessive autosomal, dominant X-linked, recessive X-linked, dominant (very rare) Applying pedigree analysis - practice

Autosomal recessive traits Trait is rare in pedigree Trait often skips generations (hidden in heterozygous carriers) Trait affects males and females equally Most common ones Cystic fibrosis Sickle cell anemia

ex. achondroplasia (a sketelal disorder causing dwarfism) Autosomal dominant traits There are few autosomal dominant human diseases (why?), but some rare traits have this inheritance pattern Only need to get the trait from one parent to be affected. ex. achondroplasia (a sketelal disorder causing dwarfism)

X-linked recessive pedigrees Trait is rare in pedigree Trait skips generations Males are more often affected than females Affected fathers DO NOT pass to their sons,

ex. Hemophilia in European royalty X-linked recessive traits ex. Hemophilia in European royalty

X-linked dominant pedigrees Trait is common in pedigree Affected fathers pass to ALL of their daughters Males and females are equally likely to be affected

Beyond Mendelian Genetics: Incomplete Dominance Mendel was lucky! Traits he chose in the pea plant showed up very clearly… One allele was dominant over another, so phenotypes were easy to recognize. But sometimes phenotypes are not very obvious…

Polygenic Traits When a character (physical feature) is influenced by several different genes, the character is Polygenic. Examples: Height, weight, color of skin, hair, and eyes Cause slight and often variable range of differences throughout population

Incomplete Dominance Snapdragon flowers come in many colors. If you cross a red snapdragon (RR) with a white snapdragon (rr) You get PINK flowers (Rr)! R R r r  Genes show incomplete dominance when the heterozygous phenotype is intermediate. R r

Incomplete dominance R r R r When F1 generation (all pink flowers) is self pollinated, the F2 generation is 1:2:1 red, pink, white R r R R R r r r R r

Codominance Two alleles affect the phenotype in separate and distinguishable ways. Neither allele can mask the other and both are expressed in the offspring and not in an “intermediate” form.

Multiple Alleles Blood Type Genes with 3 or more alleles are said to have Multiple alleles. Blood type is determined by three alleles, IA, IB, and i. The IA and IB alleles are both dominant over i, but neither IA, IB is dominant over the other (Codominance)

Environmental Impact on Phenotype pH of the soil will change the color of hydrangea flowers from blue to pink

Environmental Impact on Phenotype

Genetic Disorders Sickle Cell Anemia Cystic fibrosis Hemophilia A hereditary mutation is a mistake that is present in the DNA of virtually all body cells Genetic Disorders arise from mutations Sickle Cell Anemia Cystic fibrosis Hemophilia Huntington's Disease Hypercholesterolemia