Chapter 8Chapter 8Section 2: Mendel’s Theory

Grade 10 BiologySpring 2011

Bell RingerBell RingerDoes a tall plant plus a short

plant equal a medium plant?◦Blending

ObjectivesObjectivesDescribe the four major

hypotheses Mendel developed Define the terms homozygous,

heterozygous, genotype, and phenotype

Compare Mendel’s two laws of heredity

A Theory of HeredityA Theory of HeredityMendel’s results did not support

blending hypothesis Mendel concluded each pea plant

has two separate “heritable factors” for each character- one from each parent

Fig. 4 page 166

Mendel’s HypothesesMendel’s HypothesesMendelian theory of heredity:1.For each inherited character, an

individual has 2 copies of the gene- one from each parent

2.There are alternative versions of genes (alleles)

Mendel’s HypothesesMendel’s Hypotheses3. When two different alleles occur

together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance

4. When gametes are formed, the alleles for each gene in an individual separate independently for one another. Thus, gametes carry only one allele for each inherited character. When gametes unite during fertilization, each gamete contributes one allele.

Mendel’s HypothesesMendel’s HypothesesAlleles: different versions of a

gene ◦Purple or white flower

Mendel’s HypothesesMendel’s HypothesesDominant: expressed form of

the character◦Purple flower

Recessive: trait that was not expressed when the dominant form of the character was present ◦White flower

Mendel’s Findings in Modern Mendel’s Findings in Modern TermsTermsDominant = AARecessive = aaHomozygous: if 2 alleles of a

particular gene present in an individual are the same◦Ex. AA or aa

Heterozygous: if the alleles of a particular gene present are different◦Ex. Aa

Mendel’s Findings in Modern Mendel’s Findings in Modern TermsTermsGenotype: set of alleles that an

individual has for a character◦Ex. Aa, aa, AA

Phenotype: physical appearance of a character ◦Ex. Brown hair, purple flowers

Are phenotype and genotype related?

The Laws of HeredityThe Laws of HeredityLaw of Segregation: the two

alleles for a character segregate (separate) when gametes are formed

The Laws of HeredityThe Laws of HeredityDihybrid Cross: cross that considers two pairs of contrasting characters ◦Flower colour

and plant height

The Laws of HeredityThe Laws of HeredityLaw of Independent Assortment: the alleles of different genes separate independently of one another during gamete formation ◦Only applies to genes that are

located on different chromosomes or far apart on the same chromosome

The Laws of HeredityThe Laws of HeredityUnits of heredity are portions of

DNA called genes, which are found on the chromosomes that an individual inherits from its parents