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Mendel’s 1 st Law. The Law of Segregation Members of a pair of alleles for a given trait are separated when gametes are formed. Mendel’s 2 nd Law. The Law of Independent Assortment - PowerPoint PPT Presentation
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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