1. The Study of Heredity
Chapter 2

2. Basics of Heredity Blending Theory Gregor Mendel
Early attempt at explaining family resemblance
However, traits would not persist from generation to generation
Gregor Mendel
Augustinian friar from what is now the Czech Republic
Founder of modern genetics
Pea plant studies
demonstrated the basic concept of inheritance

3. Work of Gregor Mendel Crossed true-breeding plants to create hybrids
Hybrids – Offspring of parents that differ from each other with regard to certain traits or certain aspects of genetic makeup
Allowed the hybrid plants to self-pollinate
¾ of one trait, ¼ of the other
Dominant and recessive traits
Dominant traits are governed by an allele that can be expressed in the presence of another
Recessive traits are not expressed in heterozygotes
The principles of genetics were outlined by Mendel, but he was unaware of the biological nature of the genetic material

4. Two Principles of Mendelian Genetics
Principle of Segregation
Counters the idea of blending inheritance
For every trait, an individual has two hereditary factors, one inherited from each parent
Together, these cells determine the appearance of the trait in the organism
Principle of Independent Assortment
Inheritance patterns of differing traits are independent of one another
The distribution and selection of traits is random

5. Segregation

6. Alleles and Genes Genes Alleles
A stretch of DNA or RNA that determines a trait
Alleles
A number of alternative forms of the same gene
Ex: Gene – Eye Color; Allele – Brown, Blue, Green, etc.
Can occur at the same locus on a pair of chromosomes and influence the same trait
An allele is a fixed spot on a chromosome
Diploids – organisms with two alleles at each locus
One allele from each parent (humans)

7. Genotype and Phenotype
The genetic makeup of an individual
Can refer to an organism’s entire genetic makeup or to the alleles at a particular locus
Phenotype
The observable or detectable physical characteristics of an organism
The detectable expressions of genotypes, frequently influenced by environment

8. Simple Mendelian Inheritance
Heterozygous
Having different alleles at the same locus on members of a pair of chromosomes
Homozygous
Having the same allele at the same locus on both members of a pair of chromosomes
Recessive homozygotes – two recessive alleles
Dominant homozygotes - two dominant alleles
There are a number of human characteristics that are inherited in a simple Mendelian manner
Human earlobe can be either free-hanging (dominant) or attached (recessive)

9. Chromosomes
Genetic structure composed of a DNA molecule and associated proteins
During cell division, chromosomes consist of two strands of DNA joined at the centromere
Act as the site of assembly
Since the DNA molecules have replicated, one strand of a chromosome is an exact copy of the other

10. Human Chromosomes
Both sexes have 22 pairs of non-sex chromosomes, referred to as autosomes, and a 23rd pair of sex chromosomes
The normal female possesses two homologous sex chromosomes (XX) while the normal male has only one X chromosome which pairs with a different type, the Y chromosome

11. Cell Division Mitosis Meiosis Steps: Cell division in somatic cells
Occurs during growth and repair/replacement of tissues
The result is two identical daughter cells that are genetically identical to the original cell
Steps:
The 46 chromosomes line up in the center of the cell
The chromosomes are pulled apart at the centromere
The strands separate and move to opposite ends of the dividing cell
The cell membrane pinches in and two new cells exist
Meiosis
Cell division in specialized sex cells in ovaries and testes
Involves two divisions and results in four daughter cells, each containing only half the original number of chromosomes
These cells can develop into gametes
Recombination - the exchange of genetic material between partner chromosomes during meiosis

12. Heredity and DNA
Understanding the structure of the DNA molecule is also crucial to our comprehension of heredity
Cellular function and an organism’s inheritance depends on the structure and function of DNA
DNA is composed of two chains of nucleotides, comprising a double strand or double helix
A nucleotide consists of a sugar, a phosphate, and one of four nitrogenous bases (bases)

13. The DNA Molecule
James Watson (left) and Francis Crick in 1953 with their model of the structure of the DNA molecule.

14. DNA Structure Nucleotides form long chains
The two chains are held together by bonds formed on their bases with their complement on the other chain
Adenine(A) is the complement of Thymine(T)
Guanine(G) is the complement of Cytosine(C)
DNA bases form base pairs in a precise manner.
Adenine can bond only to thymine, etc.

15. DNA Replication The DNA molecule is able to make copies of itself
Steps:
Enzymes (specialized proteins) break the bonds between the DNA molecule
Two nucleotide chains serve as templates for the formation of a new strand of nucleotides
Unattached nucleotides pair with the appropriate complementary nucleotide

16. RNA and DNA RNA differs from DNA in three important ways:
It is usually single-stranded (This is true of the forms we discuss, but it is not true for all)
It contains a different type of sugar, ribose.
It contains the base uracil as a substitute for the DNA base thymine (Uracil is attracted to adenine, just as thymine is)

17. Protein Synthesis Process by which cells build proteins Proteins
Long chains of basic units known as amino acids
The blueprint for a specific protein is located in the DNA
Messenger RNA (mRNA) carries the genetic message
A form of RNA assembled on a sequence of DNA bases
It carries the DNA code to the ribosome during protein synthesis
Transfer RNA (tRNA) binds to one specific amino acid
Transcription
The process of coding a genetic message for proteins
A portion of the DNA unwinds and serves as a template for the formation of a mRNA strand