Traits and Inheritance Chapter 3, section 2

Explaining Mendel Mendel knew there were two sets of instructions for a trait -- genes Each instruction is an allelle Offspring get one instruction (allelle) from each parent Allelles can be dominant or recessive

Allelle Symbols Dominant alleles - capital letters Brown eyes are dominant -- B Recessive alleles - lower case letters Blue eyes are recessive -- b

Phenotype Combinations of allelles and genes affect an organism’s traits or characteristics Phenotype is the expression of the genes -- which allele is seen Phenotype is the physical appearance

Genotype Combinations of allelles (letters) that you inherit from your parents Genotypes are made of two letters Homozygous -- both allelles (letters) the same Heterozygous -- two different allelles

Examples of Genotypes and Phenotypes Homozygous Genotype HH - dominant phenotype is seen Genotype hh - recessive phenotype is seen Heterozygous Genotype Hh - dominant phenotype is seen

Predicting Offspring - Punnett Squares Mendel’s 1st Experiment Purebred Purple-flowering plant (PP) is crossed with a Purebred White-flowering plant (pp) P P p P p p P Purple flowers Purple flowers p p Ratio is 1:0 Purple flowers Purple flowers

Predicting Offspring - Punnett Squares Mendel’s 2nd Experiment Offspring (Pp) are allowed to self pollinate P p P p P P P p Purple flowers Purple flowers p p Ratio is 3:1 Purple flowers white flowers

Probability Mathematical chance something will happen Written as a fraction or a percentage Punnett squares are used to make probability predictions End for today

Other Types of Inheritance - Exceptions to Mendelian Genetics Mendel chose traits that had only two dominant or recessive alleles Inheritance can be more complex than that

Incomplete Dominance or Co dominance Sometimes the alleles don’t hide, they mix Incomplete Dominance (Co dominance) is when the two variations of a trait blend together Example - white flowers and red flowers blend to make pink flowers

Incomplete Dominance Allelles Both alleles are shown as capitals of the same letter (R and R or R and R’) The heterozygous genotype gives a blended phenotype RR = red flowers R’R’ = white flowers RR’ = pink flowers

Incomplete Dominance Punnett Squares R R R’ RR’ A red 4 o’clock (RR) plant and a white 4 o’clock (R’R’) plant are cross pollinated Pink flowers Ratio of 1:0

Multiple Alleles Sometimes there are more than 2 variations or alleles for a trait Human blood types have 3 alleles One person has 2 of the 3 alleles Two A, B, or O alleles combine to give the phenotype (bloodtype)

Polygenic Traits Traits can be controlled by more than one gene Example - shades of eye color, shades of skin color