Biology Chapter 7 Notes: Mendelian Inheritance
Gene Inheritance: Each human offspring inherits one maternal and paternal set of 23 chromosomes
Heredity: the passing of characteristics from parents to offspring through their genes; some traits are
controlled by a single gene (single trait genes); observing heredity is easy, understanding was difficult;
breeding
Mendel: what do parents give their offspring that confers similarity?
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no obvious answer at that time (1800s)
applied and tried and true methodical experimentation and scientific thinking
1. Chose a good organism to study; garden pea; easy to take care of in large numbers; short time to
breed; relatively easy to fertilize manually by pollen dusting; easy to collect hundreds of offspring from a
single cross; conduct experiments that lasted for multiple generations
2. Focused on categorized traits; wrinkled or round shape of seed; yellow or green in colour
3. Began by repeatedly breeding together similar plants until he had many distinct populations (truebreeding); after his true-breeding plants, Mendel began a straightforward process of experimentation
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purple-flower trait dominant; white-flower trait recessive
1. Parents put into every sperm or egg it makes a single set of instructions for building the trait (gene)
2. Offspring receive two copies of the instructions from any trait
3. Trait observed in an individual depends on the two copies of the gene it inherits from its parents
Homozygous: individual inherits the same two alleles for this gene
Heterozygous: individual inherits different allele from each parent
Mendel's Law of Segregation: two copies of each individual's gene is contributed from the two sets of
alleles from the mother and father
Phenotype: visible or physical features / Genotype: genetic composition of the organism
Unable to deduce an individual's genotype from observing the phenotypes
Punnett Square: a tool to analyze and predict the outcome of a cross between two individuals
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genes on same chromosomes are sometimes inherited together; red hair and freckles;
sometimes alleles for two genes are inherited together and expressed almost as a package deal;
some genes are close together on a chromosome called linked genes; during the production of
gametes, the crossing of the parental/maternal genes end up interacting
Probability and Chance
Aa X aa = 0.5 x 1.0 = 0.5 of the recessive gene to be passed (albinism); Tt X Tt = 0.5 X 0.5 = 0.25 of
recessive gene passed (Tay Sachs)
Test Cross: test out the uncertainty between the genotypes of the organism; genes are invisible but their
identity can be revealed by a test-cross; Albino X Non-albino will be test crossed to determine if the nonalbino is a homozygous dominant or a heterozygous
Pedigrees
Square: Male
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Circle: Female
Shaded: Trait of interest
Blank: No trait of interest
used to decipher and predict the inheritance patterns of genes
type of family tree that can help answer questions related to genetic disease/other condition
inheritance
information is gathered from many related individuals across multiple generations
Traits that are controlled by genes on the sex chromosomes are called sex-linked traits
AA, Aa or aA
enables identification of individuals who are carriers of a recessive trait
Incomplete & Codominance
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not all traits are completely dominant or completely recessive
Incomplete dominance: an intermediate product is produced when two true-breeding
organisms are crossed; white CwCw X red CrCr snapedragons crossed turn into pink CwCr
Codominance: a heterozygous individual displays characteristics of both homozygotes; sickle-cell
disease will show both normal red blood cells and those under sickle conditions
Two Allele Genes
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blood groups are an example of multiple allelism; ABO
A & B are codominant with each other; A has A antigens and B has B antigens AB have antigens
that produce antibodies that attack other blood antigens
O does not have antigens; produces antibodies that attack A and B antigens; universal donor (O
Neg)
AB is universal recipient
Rh blood group marker effects a single gene with two alleles; Rh marker alleles are dominant;
without Rh markers are recessive; Rh negative can not receive from Rh positive but Rh positive
can receive from Rh negative
Influence of Genes
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some traits like height (one on chromosome 15 which helps testosterone convert to estrogen
which stops bone growth)are influenced by many different genes; said to be polygeneic
additive effects: describes what happens when the effects of alleles from multiple genes all
contribute to the ultimate phenotype
skin colour, eye colour, behaviour traits
pleiotropy: some genes can influence multiple unrelated traits (can be true for nearly all genes);
the sickle gene in a heterozygous state can effect resistence to malaria parasite
SRY gene (sex-determining region on the Y chromosome): causes fetal gonads to develop as
testes shortly after fertilization; secretion of testosterone, cascade of other developmental
changes, internal and external male reproductive structures
Sex-linked Traits
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colour blindness: x chromosomes have genes that make proteins to properly sense light; there is
a non-functioning version of the protein which is recessive but it only takes one x chromosome
from women to pass it onto men because the Y chromosome does not carry this gene
Environmental Effects
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PKU: mutant gene that does not allow the conversion of phenylalanine into trosine
Siamese cats produce darker furs in colder temperatures;