Chapter 10 Genetics: Mendel and Beyond Biology 101 Tri-County Technical College Pendleton, SC

It’s a character trait  A character is a feature such as flower color Best described as a gene  A trait is a particular form of a character, such as white flowers Best described as an allele  A heritable character trait is one that is passed from parent to offspring  Before Mendel, blending was model of choice…and it was logical to a point

Law of Segregation  Mendel and it’s all about flowers and sex  In sexually reproducing organisms, an individual possesses 2 “factors” for each trait One came from mommy and one from daddy  When that individual makes gametes, those “factors” (alleles) will separate (segregate) from each other and wind up in different gametes  **Gamete receives only ONE member of the pair of alleles

Defining Definitions  Genotype is genetic constitution of the organism It’s what in the genes  Phenotype is the physical expression of the genotype It’s what one sees—phenotype is the expression of genotype  Dominant means always expressed (in the phenotype)  Only one allele needed for its expression  DOES NOT MEAN “THE BEST TO HAVE”

Definitions, cont.  Recessive means “covered”, “hidden”, or “masked” by presence of dominant allele Recessive allele only expressed when present in pairs  Homozygous means “same” or two copies of same allele  Can be homozygous dominant or homozygous recessive  TT or tt, respectively

Definitions, cont.  Heterozygous means “different” or “other”  Two different alleles of the “trait” (gene/allele) in question (Tt)  Monohybrid means differing in only ONE trait tall or short; yellow or green, etc.  Dihybrid means differing in TWO traits tall and yellow; short and green

Monohybrid Cross  P = parental generation  F 1 = first filial generation (first generation of hybrid)  F 2 = second filial generation (second generation of a hybrid)  Punnett square is an application that allows prediction of probability of genotypes/ phenotypes from a genetic cross

Test Cross  Test cross is dated concept that still serves useful purpose  Estes and the field of corn  Defined as mating an individual of unknown genotype with an individual homozygous recessive for the trait  Figure the Punnett square for this one…you will see it again!!

Law of Independent Assortment  In dihybrid crosses, parental traits appear in new combinations of 2 of the phenotypic classes such new combinations called recombinant phenotypes  Alleles of different genes assort independently of one another during gamete formation In other words, T does not always stay with Y and vice versa  Law of Independent Assortment NOT as universal as law of segregation

Independent Assortment, cont.  Applies to genes on separate chromosomes but not necessarily to those that lie on same chromosome  Simply states that chromosomes segregate independently during formation of gametes as do any two genes (alleles) on separate chromosome pairs

Dihybrid Crosses  Review: dihybrid means two contrasting traits  TTYY crosses with ttyy  Do the Punnett square and look for 9 genotypes and 4 phenotypes  Be sure and remember the phenotypic ratio…you will see it again and again!!!

Incomplete Dominance  When heterozygous phenotype is intermediate, gene is said to be governed by incomplete dominance/lack of dominance Looks like blending to be sure Time for a Wally World story…Yeah!!!!  Red and white snapdragon cross produces all pink offspring All F 1 to self-fertilize and predict F 2 Black rooster and white hen = all gray chicks Red bull and white cow = all roan calves

To dominate…or Not  Complete dominance results when single allele produces enough protein to give maximum phenotypic response  Incomplete dominance results when heterozygotes show intermediate phenotype  Codominance results when two alleles at locus produce two different phenotypes that both appear in heterozygoes  Means both are fully expressed  Best example is ABO blood grouping

ABO Blood Grouping  Blood type determined by proteins on surface of RBCs  Protein coded for by info in DNA  Alleles are I A and I B  There are six genotypes and four phenotypes  This is ABO only, we will discuss Rh factor  This one always gets me into trouble  Chalk talk time on ABO and Rh typing

Linkage  Association between markers (genes/alleles) on same chromosome such that they do NOT show random assortment and seldom recombine  Closer the markers, lower frequency of recombination  Closer they are together, more likely they are to STAY together  MAP units

Determining Sex  Sex (gender) determined by sex chromosomes (23 rd pair)  Females are XX and males are Xy  AFAWK, only thing on y chromosome is SRY (male-determining) gene and gene for hairy ears  Nature was not overly kind to males  Implications of XX and Xy

Sex-linked Disorders  Defined as being “tied” to X sex chromosome  Sex-linked disorders affect males more  One mutant recessive allele leads to red- green colorblindness, muscular dystrophy, and hemophilia  Human mutations inherited as sex-linked dominant must LESS common than recessives

Sex-linked, cont.  Dominants appear in every generation and individuals carrying harmful mutation (even as heterozygotes) often fail to survive and/or reproduce  Chalk talk time on hemophilia and/or color blindness