mendel and heredity chapter 8 ms. hogg, biology. the origins of genetics heredity – the passing of...
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Mendel and
HeredityChapter 8
Ms. Hogg, Biology
The Origins of Genetics
• Heredity – the passing of characteristics from parent to offspring– Before DNA and chromosomes were discovered,
heredity was one of the greatest mysteries of science!
Gregor Mendel• The scientific study of heredity began with
Gregor Mendel– Austrian monk– Carried out experiments with garden peas– He was the 1st to develop rules that accurately
predict patterns of heredity.– “Father of Genetics”
Pea Plant
7 Characteristics Mendel Studied:
Pollen Transfer in Mendel’s Experiments
Mendel’s Hypotheses#1
• For each inherited characteristic, an individual has two copies of the gene – one from each parent.
Mendel’s Hypotheses#2
• There are alternative versions of genes.– These different “versions” are called alleles.– An individual receives one allele from each parent.
Mendel’s Hypotheses#3
• When two different alleles occur together, one of them may be completely expressed, while the other may have no observable affect on the organisms appearance.
– Dominant = expressed trait– Recessive = trait that is NOT expressed
Mendel’s Hypotheses#4
• When gametes are formed, the alleles for each gene in an individual separate independently of one another.
Terms of Genetics
• Homozygous = two alleles of a gene are the SAME– Example: BB or bb
• Heterozygous = two alleles of a gene are DIFFERENT– Example: Bb
• Genotype = the set of alleles that an individual has for a trait– Example: BB
• Phenotype = the physical appearance of a characteristic – Example = Brown hair
Terms of Genetics
Punnett Square
• A punnett square is a diagram that predicts the outcome of a genetic cross
• Monohybrid cross:
The Hardy-Weinberg Principle
p2 + 2pq + q2 = 1
You can use this equation to predict genotype frequencies in a
population.
The Hardy-Weinberg Principle
p2 + 2pq + q2 = 1
The sum of allele frequencies must always equal 1.
p + q = 1
Exceptions…
• The Hardy-Weinberg principle holds true for any population as long as evolutionary forces are not acting…– Mutations– Gene flow– Nonrandom mating– Genetic drift– Natural selection
A Review…Monohybrid Cross
A Review…Dihybrid Cross
NOTE: Must use the “FOIL” method to correctly set up the parent genotype combinations on the Punnett Square
Incomplete Dominance
• An organism can display a phenotype that is intermediate between the two parents
• Example: Snapdragon flowersRR = redRr = pinkrr = white
Incomplete Dominance
Codominance
• Two dominant alleles can be expressed at the same time
• Example: Human Blood groups (ABO blood groups)
Type A = IAIA or IAi Type B = IBIB or IBiType AB = IAIB (Universal Recipient)Type O = ii (Universal Donor)
Codominance – ABO Blood Groups
Rh FactorSource: NobelPrize.org
Human Blood Type FrequencySource: RedCrossBlood.org
Sex-linked Traits• In sex-linked inheritance, the gene responsible
for the disease is located on the X chromosome.
• Usually, the abnormal gene is recessive. – For these reasons, the resultant disorder is called
an X-linked recessive disease
Sex-linked Disorders
• Color blindness– A person with the most common form of color
blindness cannot distinguish red from green hues.
• Hemophilia– In hemophilia, the blood does not clot properly.
Color blindness Tests
Hemophilia
Pedigrees
Polygenic Traits
• Polygenic traits are controlled by two or more genes, usually on different chromosomes.
• Examples of Polygenic Traits:– Height, skin color, weight, – Many common diseases, such as heart disease,
are caused by polygenic genes and environmental factors (Lifestyle choices such as diet and exercise)
Human skin color is controlled by at least eight different loci on different chromosomes...
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