1. Mendelian and Molecular Genetics
Chapter 26
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2. DNA DNA double molecule (double helix)
Resembles twisted ladder
Sugar and phosphate groups make up the backbone
Two strands linked by nitrogenous bases
Complementary bases: A with T, G with C
Length measured in numbers of base pairs
Hydrogen bonds help hold strands together

3. James Watson and Francis Crick DNA Molecule - 1953

4. Genetics and Meiosis
Gene – portion of DNA that determines a characteristic or trait
Locus – site on a chromosome where a particular gene is located
During meiosis diploid cell (2n) converted to haploid cell (n)
Diploid 2n  meiosis  haploid gamete n
Diploid organisms
2 sets of chromosomes
One set from each parent
2 genes for each characteristic
One gene from each parent

5. Single Gene Inheritance Patterns
Alleles- alternative forms of a gene
Located on the same pair of homologous chromosomes at same locus
Genome – set of all genes necessary to specify an organism’s complete list of characteristics
Genotype – all genes present in the organism
Consists of DNA code
Cannot see genotype
Phenotype - observable characteristics of an organism
Determined by the ways in which combination of alleles express themselves

6. Genotype: EE Ee ee Phenotype: 2 alleles for free earlobes (EE)
2 alleles for attached earlobes (ee)
1 allele for free earlobes, 1 for attached (Ee)
Phenotype:
Two phenotypes
Alleles
Genotype
Phenotype
E = free earlobes
e = attached earlobes EE
Free earlobes Ee ee
Attached earlobes

7. Gregor Mendel
Austrian monk who performed experiments that showed inheritance patters of certain characteristics in pea plants
Studied only one trait at a time
Concluded which traits were dominant and which were recessive
Father of Genetics

8. Mendel’s Pea Plants

9. Dominant- recessive cross
Male genotype Tt
Possible male gametes T and t
Female genotype Tt
Possible female gametes T and t T t

10. More complex models Phenotypes usually determined by:
type of allele
the alternative ways they interact with each other
how environment influences their expression
Different patterns seen:
Incomplete dominance
Multiple alleles
X-linked characteristics
Polygenic inheritance

11. Incomplete Dominance Alleles lack dominant relationships
Heterozygous offspring have intermediate phenotype not distinct from either homozygous parent
Snapdragons
Two alleles (white and red petals)
Neither recessive
Allele for white petal FW
Allele for red petal FR
Genotype
Phenotype
FWFW
White flower
FRFR
Red flower
FRFW
Pink flower
Both red and white flower alleles partially express themselves
Two different alleles, three phenotypes

13. Multiple Alleles IAIA IAi IBIB IBi IAIB ii
Multiple alleles – all forms of the same gene that control a trait
More than two different alleles for a single trait
One organism can have a maximum of two of the alleles
Example: Blood type
Three alleles A, B, O
A and B show codominance
A and B dominant over O
Genotype
Phenotype
IAIA
Blood type A
IAi
IBIB
Blood type B
IBi
IAIB
Blood type AB ii
Blood type O
Allele IA
A antigens IB
B antigens i
Neither A or B antigens

15. X-Linked Genes
Alleles located on same chromosome tend to be inherited together (linked)
Closer they are, more likely they are linked
Sex chromosomes: X and Y chromosomes
Female XX, Male XY
Y much shorter than the X chromosome
Does not have many of the alleles that are found on the comparable portion of the x chromosome
In males, the presence of a single allele on the X chromosome will be expressed no matter if it is dominant or recessive
X-linked genes - found together on x chromosomes
More than 100 genes
color deficiency, hemophilia, muscular dystrophy, brown teeth
Y-linked gene
SRY controls the differentiation of the embryonic gonad to a testis

17. Polygenic Inheritance
Characteristics determined by interaction of genes at several different loci
Number of different pairs of alleles determine a characteristic