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Mendel and the Gene idea (chapter 14) (Inheritance patterns are often more…
Mendel and the Gene idea (chapter 14)
Mendel uses the scientific approach to identify two laws of inheritance (14.1)
He conducted his experiment by:
waited for pollinated carpel to mature into pod
planted seeds from pod
transferred sperm-bearing pollen from stamens of white flower to egg bearing carpel of purple flower
examined offspring (which gave all purple flowers)
Mendel removed stamens from purple flowers
4 main ideas of the inheritance model created by mendel
For each character, an organism inherits two alleles, one from each parent
The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation
Or the two alleles at a locus may differ, as in the F1 hybrids
If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant
Alternative versions of genes account for variations in inherited characters
These alternative versions of a gene are called alleles
Each gene resides at a specific locus on a specific chromosome
(the law of segregation): the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
Meiosis is the name of the process from which the law of segregation applies
Gregor Mendel was a scientist who lived in 1822-1884 and discovered the fundamental laws of inheritance
He deduced that genes come in pairs and are inherited as distinct units, one from each parent.
Mendel tracked the segregation of parental genes and their appearance in the offspring as dominant or recessive traits
Heterozygotes
An organism with two different alleles for a gene is a heterozygote and is said to be heterozygous for the gene controlling that character
. Unlike homozygotes, heterozygotes are not true-breeding
homozygote
An organism with two identical alleles for a character is called a homozygote
It is said to be homozygous for the gene controlling that character
Probability laws govern Mendelian Inheritance (14.2)
Addition rule
the probability that any one of two or more mutually exclusive events will occur is calculated by adding together their individual probabilities
The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous
conclusion
We can apply the rules of probability to predict the outcome of crosses involving multiple characters
multiplication rule
the probability that two or more independent events will occur together is the product of their individual probabilities
Segregation in a heterozygous plant is like flipping a coin: Each gamete has a ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele
evaluation
A multi character cross is equivalent to two or more independent monohybrid crosses occurring simultaneously
Results of a monohybrid
3:1 ratio
Results of a dihybrid
9:3 3:1 ratio
Inheritance patterns are often more complex than predicted by simple Mendelian genetics (14.3)
codominance
two dominant alleles affect the phenotype in separate, distinguishable ways
a person with type AB blood, which means that both the A allele and the B allele are equally expressed
multiple alleles
in the population some genes have more than two alleles
An excellent example of multiple allele inheritance is human blood type. Blood type exists as four possible phenotypes: A, B, AB, & O.
incomplete dominance
the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
RR= red, rr= white, Rr= pink
pleiotropy
One gene affects multiple phenotypic characters
ex. sickle cell disease
Complete dominance
occurs when phenotypes of the heterozygote and dominant homozygote are identical
PP and Pp both give purple flowers
Epistasis
the phenotypic expression of one gene affects the expression of another gene
example
For example, in Labrador retrievers and many other mammals, coat color depends on two genes
One gene determines the pigment color (with alleles B for black and b for brown)
The other gene (with alleles E for color and e for no color) determines whether the pigment will be deposited in the hair
polygenic inheritance
a single phenotypic character is affected by two or more genes
Examples of human polygenic inheritance are height, skin color, eye color and weight
Many human traits follow Mendelian patterns of inheritance (14.4)
Recessively inherited disorders
Tay Sachs disease
sickle cell anemia
Dominantly inherited disorders
Marfan syndrome
Huntington's disease
Humans are not good subjects for genetic research
Generation time is too long
Breeding experiments are unacceptable
Parents produce relatively few offspring
However, A pedigree is a family tree that describes the interrelationships of parents and children across generations is acceptable
fetal testing
In amniocentesis, the liquid that bathes the fetus is removed and tested
In chorionic villus sampling (CVS), a sample of the placenta is removed and tested