Transcript Mendel and Heredity

Mendel and Heredity
Chapter 10 and 12
Key Terms
• Heredity – The passing of traits from parents to
offspring
• Genetics – the branch of science that studies
heredity
• Gene – the unit of heredity – codes for a
specific trait
• Gregor Mendel – the father of modern genetics
• Gametes – male and female sex cells
• Zygote – forms when the sperm and egg meet
Summary of Mendel’s
Experiments
• Why peas?
– Characters exist in two clear forms (tall or
short, purple or white flowers, etc.)
– Self-fertilization is possible
– Small plant
– Grows easily
– Matures quickly
– Many offspring
What did Mendel do?
• He self pollinated the pea plants until they were
“true-breeding”
– Had only one version of a trait in their genes
– Only genes for purple flowers, or for being tall
– This made the “P-generation” (parent generation)
• He then crossed 2 different true-breeding plants
and recording the results. This made the F1
generation (filial 1). These are hybrids.
• Finally, he crossed 2 members of the F1
generation making the F2 generation (filial 2)
What was the result of Mendel’s experiment?
•All members of the F1
generation displayed the
dominant traits
•Seemed as if one trait
had disappeared!
•The members of the F2
generation displayed a 3:1
ratio of dominant to
recessive
•The trait reappeared!
The Result of Mendel’s Experiments.
Mendel’s Hypotheses:
– For each inherited character, a person has 2
copies of the gene – one from each parent
– There are alternative versions of genes
(tall/short, wrinkled/round, etc.)
– When 2 alternative versions occur in the same
individual, one is expressed and the other is
hidden
Genetic Rules
• Rule of Unit Factor:
– There are genes on chromosomes that control
traits and at least 2 versions of each trait exists
• Rule of Dominance
– One trait is dominant to the other and masks the
other gene when it is present
• Law of Segregation
– Traits are inherited independent of each other
• Law of Independent Assortment
– Homologous chromosomes sort into the
gametes independently of each other
Some more terms
• Alleles –different versions of a gene
• Dominant – the gene that is always
expressed if it is present – capital letter
• Recessive – must have 2 copies to be
expressed, otherwise it is hidden by the
dominant gene – lower-case letter
• Homozygous – person has 2 of the same
allele – TT or BB or tt or bb
• Heterozygous – person has 2 different
alleles – Tt or Bb
Phenotype
• The way an organism
looks
• Only reveals the
genotype if the
recessive trait is
displayed
Genotype
• The genes that an
organism possesses
• Homozygous
• Heterozygous
How do we determine the genotype
and phenotype of the offspring
Punnett Squares!
• Used to determine
probability
• Monohybrid
• Dihybrid
• Trihybrid
More Complex Inheritance
• Incomplete Dominance – the heterozygote has a
blend of the 2 traits
• Codominance – both traits occur equally
• Multiple Alleles – more than two alleles for a
trait
• Polygenic Inheritance – more than one gene
controls the trait – usually shows a wide range of
variation – skin color and height
Nature vs. Nurture
The effect of the environment
• Environmental effects can determine if a
gene is expressed
– Chemicals, stress, sunlight, nutrition, etc
• Internal environment can also determine if a
gene is expressed
– Gender, weight, hormone imbalances
Sex-linked Genes
• On the X or Y Chromosome
• Examples: Color-blindness, hemophilia,
duchene’s muscular dystrophy
• Disorders occur more frequently in males
Codominance
Incomplete Dominance
Multiple alleles
Semi Dominance