Mendelian Genetics Introduction to Genetics

Gregor Mendel  Father of Genetics  GENETICS: study of heredity  HEREDITY: passing of traits from parent to offspring  TRAITS: characteristics  Gregor learned about these patterns by studying PEAS

Mendel’s Pea Plants  Pea plants are normally self- pollinating (fertilizes itself; producing offspring from a single parent)  Self-pollinating plants produce offspring identical to the parent.  Mendel eliminated the self- pollinating and allowed them to cross-breed so he could observe the offspring of 2 different parents.

Medel’s Pea Plants  Mendel Studied 7 different traits and looked at 3 generations.  P1= the original parent generation  F1= first generation of offspring  F2= second generation of offspring

Mendel’s Conclusions  Inheritance is determined by genes that determine the traits AND there are 2 different forms of a gene, called alleles.  Some alleles are DOMINANT and some are RECESSIVE

Review: How do traits get passed from parent to offspring?  Through GAMETES: sex cells  Male gamete is the SPERM  Female gamete is the EGG  FERTILIZATION: union of egg & sperm  ZYGOTE: fertilized egg

GENES VS. ALLELES Genes  Segment of DNA that codes for one protein/trait  Most traits are determined by TWO genes.  Remember chromosomes have a homologous match…two chromosomes with similar genetic information  EXAMPLE: Hair color, eye color, height in pea plants Alleles  Each form of a gene is called an ALLELE  EXAMPLE: brown or blond blue, hazel, brown tall, short

Law of Dominance DOMINANT (allele)  Always expressed if present  Only need ONE dominant allele to be expressed EXAMPLES: dark hair, brown eyes, tall peas RECESSIVE (allele)  May be present but will not be expressed if paired w/ dominant allele  Requires TWO recessive alleles for recessive trait to be expressed EXAMPLES: light hair, blue eyes, short peas

Law of Segregation  Gene pairs separate during formation of gametes (meiosis)  Each gamete gets ONE allele from a pair  Each parent passes down one allele to their offspring  Offspring get 2 alleles (one from each parent)

Preparing for Genetic Crosses  Letters will represent alleles  Use the first letter of the dominant trait  Capital letter = dominant trait  Lower case letter = recessive trait  EXAMPLE: height in pea plants TALL is dominant over short, so… T = tallt = short

Genotype vs. Phenotype  GENOTYPE: actual genes in the pair; can’t see…they’re on chromosomes…but they determine the individual’s traits  Use terms: homozygous: both genes the same (TT, tt) heterozygous: genes different; 1 dominant, 1 recessive (Tt, tT) All possible combinations using T, t: TT, tt, Tt, tT

Genotype vs. Penotype  PHENOTYPE: the trait that is expressed; the ‘physical’ appearance Phenotype Genotype  TT…tall (homozygous dom)  tt….short (homozygous rec)  Tt….tall (heterozygous)  tT….tall (heterozygous)

Mendelian Traits  Traits that have 2 alleles  EXAMPLE: cleft chin  If a person inherits two dominant alleles (CC) or one dominant & one recessive (Cc), they will have a cleft chin (top)  If a person inherits 2 recessive alleles (cc), they will not have a cleft chin (bottom)

Solving Genetic Crosses  Use PUNNETT SQUARES  Diagram used to predict the offspring given genotypes of parents.  The parents genotypes go on outside of square (one parents’ across top; other parents’ down left side

Monohybrid Cross  These involve just ONE trait; therefore, you will see just ONE letter (but it can vary in size).  Ex. Aa x Aa or AA x aa

Test Cross  Just because you know the phenotype, does not mean you know the genotype for a particular trait!  To identify the genotype of an organism with a dominant trait, you must perform a test cross.  This means you mate the organism of the unknown genotype with a homozygous recessive organism.  Ex. You need to find the genotype of a brown rat. Brown is dominant to white. The genotype of the brown rat could be _____ or _______.