Mendel’s 1st Law

• The Law of Segregation– Members of a pair of

alleles for a given trait are separated when gametes are formed

Mendel’s 2nd Law

• The Law of Independent Assortment– When two or more pairs of characteristics are

considered at one time, each pair shows dominance and segregation independently of the other

– Eg. A plant that is heterozygous for two pairs of alleles, Tt Rr, can form four types of gametes:• TR, Tr, tR, and tr

Incomplete Dominance

• There are instances in nature where strict dominance did not apply

• In snapdragons, crossing of white-flowered plant with red-flowered plant produces pink-flowered plants

Incomplete Dominance

• Dominance:– The dominant allele (A) can hide the expression of

a recessive allele (a) in a heterozygous condition• Incomplete Dominance:– Different alleles of a gene are expressed in the

heterozygous condition to produce an intermediate phenotype

– When neither gene is completely dominant over the other

Having two RR genes produce enough red pigment to make red flowers

Having only 1 R genes produce only enough red pigment to make pink flowers

Co-dominance

• Two alleles expressed at the same time• One parent is homozygous red (RR) and the

other homozygous white (rr)– Offspring (Rr) is a blend red and white

Co-dominance

• Each individual hair is either completely white or completely red

• NOT intermediate in colour

• Neither allele dominates the expression of the other

Blood Type

• Human blood types show co-dominance• Possible blood types:– A, B, AB, and O

• Blood type is determined by a pair of alleles• There are 3 different alleles (A, B, O) that may

be found at the locus on either of the homologous chromosome

• Multiple allelism

Blood Type

• Alleles for A = IA

• Alleles for B = IB

• Alleles for O = i• IA and IB are co-dominant over i

Genotype Phenotype

IAIA or IAi Type A

IBIB or IBi Type B

IAIB Type AB

ii Type O

Sex Linkage

• Human sex is determined by X and Y chromosomes

• Male = XY• Female = XX

• Any gene on the X chromosome, whether dominant or recessive, is expressed in males

• Traits controlled by genes on the X chromosome are called X-linked traits

X-Linked Disorders

• Colourblindness• Muscular Dystrophy• Hemophilia• Fragile X Syndrome

Punnett Square

• Use X’s and Y’s to represent male and female• Use superscript letters to designate the alleles• Eg.

• In fruit flies, the allele for eye colour is located on the X chromosome, where red eyes is dominant to white eyes. Show the cross of a heterozygous red-eyed female and a white eyed male.

Quest on Friday

• Out of 40

• Importance of cell division• Stages of cell division– mitosis• Prophase,metaphase,anaphase, telophase• Know diagrams and descriptions

• Meiosis– Crossing over– Stages– location

• Problems with meiosis– non disjunction– Aneuploidy– Monosomy, trisomy

• Chromosomal Mutations– Deletion, Duplication, Inversion, Translocation

• Sexual vs Asexual reproduction– Advantages and disadvantages of both

• Types of asexual reproduction– Diagrams and descriptions

• Mendel & experiments• Mendel’s Phenotypes vs Genotypes ratios F1

and F2 generation• Homozygous vs heterozygous• Law of independent assortment and law of

segregation

• Punnett squares• Incomplete dominance• Co dominance• Multiple alleles• Sex linked traits