Chapter 10 Mendel, Genes and Inheritance
10.1 The beginning of Genetics: Mendel’s Garden Peas
Blending theory of Inheritance: popular belief which suggested that hereditary traits blend
evenly in offspring through mixing of the parents’ blood, i.e. like coffee and cream
Mendel studied a variety of heritable characteristics called characters
o Characters = traits
Mendel established that traits are passed to offspring in the form of discrete hereditary factors
which are now known as genes
Mendel observed that rather than blending evenly many parental traits appeared uncharged in
offspring, whereas some disappeared in some generations and reappeared in other
10.1a Mendel Chose True-Breeding Garden Peas for His Experiment
Mendel chose the garden pea for his research because the plant could be grown easily in the
monastery garden
The male gametes are sperm found in the pollen, female gamete found in the carpel
Pea plants self-fertilize or self-pollinate, self
Cross-pollination: pollen to fertilize these flower then had to come from a different plant
True-breeding: when selfed they passed on traits without changes from one generation to the
next
10.1b Mendel First Worked with Single Character Crosses
Would traits blend evenly?
o Mendel took pollen from purple flowers and put them with white flowers
o Preformed the reciprocal cross
o Seeds were the product of the cross
F1 generation: first generation of offspring
P generation: parental generation, initial cross
F2 generation: second generation
In the F1, there was only purple flower
In the F2, there was a 3:1 ratio of purple to white flowers
In all cases he observed a uniform F1 generation, in the F2 the missing trait reappeared in
definite predictable proportions among the F2 offspring
10.1c Mendel’s Single-Character Cross led to the Principle of Segregation
Mendel’s factors are genes located on chromosomes
Alleles: different versions of a gene that produces different traits of a character
o Two allele for a gene
o Organisms with two copies of a gene are called diploid
Mendel deduced that the trait that appeared in the F1 was the stronger allele, calling it the
dominant allele
‘If an individual’s pair of gene consists of different alleles one allele is dominant over the other,
recessive allele
If an organism carries two different alleles, the dominant allele is the one that determines the
appearance of the organism
The dominant allele doesn’t directly inhibit the recessive alleles Principle of Segregation: The pairs of alleles that control a character segregate as gametes are
formed, half the gametes carry one allele and the other half carry the other allele
o The fusion of egg and sperm creates a zygote, the zygote receives one allele from the
male gamete and one from the female gamete
Homozygote: containing one kind of allele
Homozygous: containing same allele for one gene, rr or RR
Heterozygote: contain two different allele for a gene, said to be heterozygous for that trait
Monohybrid: an F1 heterozygote produced from a genetic cross that involves a single trait
Monohybrid Cross: a genetic cross between two individuals that are each heterozygous for the
same pair of alleles
Genotype: genetic constitution of an organism
Phenotype: out ward appearance of the genes
Mendel’s Crosses proved
o 1) the genes that govern genetic characters are present in two copies in individuals
o 2) if different alleles are present in a pair of genes, one is allele is dominant over the
other
o 3) The two alleles of gene segregate and enter gametes singly
10.1d Mendel Could Predict Both Classes and Proportions of Offspring from His Hypothesis
Product Rule of Probability
o When two of more events are independent, the probability that they will both occur is
calculated using the product rule
o Product Rule: individual probabilities are multiplied
o Ex. Probability that your 4 children may be all girls
½ x ½ x ½ x ½ = 1/16
The Sum Rule in Probability
o Sum Rule: when several different events all give the same outcome, that is, the
probability that either event A or B or C will occur equals the probability of event A plus
the probability of event B and the probability of event C
Probability in Mendel’s Crosses
o Producing a PP zygote is from a monohybrid is ½
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