introduction to genetics & dna - mcclurg science · introduction to genetics & dna {dna....
TRANSCRIPT
Introduction to
Genetics & DNA
{DNA
What is Heredity?
• Heredity explains how traits may be passed from parents to offspring.
Heredity•People have bred plants and animals for thousands of years to get desirable traits.
•No one understood how traits were transmitted until Gregor Mendel.
Chromosomes•This is a chromosome.
•Chromosomes contain hundreds of traits (genes) on each chromosome.
Chromosomes
Humans have 46 chromosomes in 23 matching (homologous) pairs.•One member of each pair came from the mother & one from the father.
What is Genetics?
• Genetics is the study of Heredity. –How traits are transmitted in sexually reproducing species.
–How genetic variations can occur.
–Why offspring are not exactly like their parents or siblings.
What is Genetics?
• Genetics is not caused by…–Accidents–Microbes–Learned traits
Inherited Traits help with Survival
They offer protection from predators OR make an animal a more effective predator
•Traits include–Intelligence–Good senses–Camouflage–Quick speed–Herding behaviors–Hibernation
GenesGenes are on chromosomes.• The genetic material (chromosomes) is contained in the nucleus of each cell• Gametes (sex cells) formed in meiosis contain half the genetic material.• This genetic material can then be passed onto offspring.
Alleles for Genes Separate
Alleles: each individual has two alleles for each trait (gene).• Mendel’s Law of Segregation: during meiosis, the two alleles for each
trait separate independently into different gametes (egg or sperm).
Gregor Mendel
• Mendel was a Czechoslovakian monk (1822-1884).• He tended the garden at the monastery.• Published scientific papers on genetics.• Mendel is considered the Father of Genetics.
Mendel’s SubjectsMendel worked with pea plants that could have ONLY TWO forms of a trait.
Mendel’s Subjects
Mendel ALWAYS started with PUREBRED plants • (plants that with the same trait or character that had been bred together for many generations).
• Mendel bred plants that had two alternate forms (alleles) of the same trait or character.
–For instance, smooth seeds (S) were crossed with wrinkled seeds (s).
Mendel’s success• Counting the offspring from many crosses.
• Using mathematics to analyze the trends in his data.
Mendel’s First Experiment
• Mendel always started by crossing purebred plants with two alternate characters
P1 x P2 → F1
• P = Parental purebred type & F = Filial generation (offspring)• Use a Punnett Square to predict the offspring.
x
→ ?
Punnett Square: P1 x P2 → F1
The F1 offspring ALWAYS had the trait of ONLY ONE
parent.
W W
W
w
w
Ww w
W W
W W
w w
w w
Dominant Traits• Dominant traits appear when one dominant allele is present.• The trait that appeared in the all of the F1 (first generation of offspring) was
called the Dominant trait. For example:– Purple flower x white flower → purple flower– Green seeds x yellow seeds → green seeds – Smooth seeds x wrinkled seeds → smooth
• Law of Dominance: the dominant trait (allele or phenotype) always shows whenever it is present.
• Recessive: the trait (allele or phenotype) that shows when no other trait is present.
If all F1 offspring were alike…
What happened to the other parent’s trait (recessive white flower) ?
What happened to the other trait?
Mendel then crossed all of the F1 (purple flowered) offspring with each other: F1 x F1→ F2
The alternate trait reappeared among the F2 offspring!
Hypothesis: traits are genes
After thousands of crosses, scientists came up with the idea of a gene.
Not these genes!
Pairs of genes: Terminology
Alleles for genes (traits) come in pairs
Homozygous: having two of the same allelesHeterozygous: having two different alleles
PPHomozygous
Dominant
ppHomozygous
Recessive
PpHeterozygous
Recessive/Dominant Inheritance
Affected Individuals
Recessive TraitsRecessive traits can be carried in a person's genes without appearing in that person. For example, a dark-haired person may have one gene for dark hair, which is a dominant trait, and one gene for light hair, which is recessive.
• Albinism (a) : the inability to produce pigment in the skin, hair, and eyes.
Many species can have albinos!
Other recessive traits• Pinto horses• White tigers• Horned cattle
Dominate TraitsA dominant trait is opposed to a recessive trait which is expressed only when two copies of the gene are present. (In genetic terms, a recessive trait is one that is phenotypically expressed only in homozygotes).
Homozygous dominant, WWW = White
Heterozygous sheepWw, w = blackSheep will show dominant trait and be white
Dominant Traits
Polydactyly (P): having more than 5 digits on a limb.
Dominant Traits
Shortened digits–Brachydactyly (B)
Dominant Traits
Achondroplasia (A - dwarfism)
These dwarfs have shortened limbs, but normal heads & torsos.
Homozygous dominant fetuses (AA) die early in development.
Incomplete Dominance TraitsIncomplete dominance happens when each genotype has its own individual phenotype, which causes neither allele to truly dominate the other, either because neither are dominant or because the dominant allele fails to dominate the recessive allele fully.
Codominant Traits
Codominance is a relationship between two versions of a gene where each allele is equally dominated
Sex-Influenced traits
Sex-influenced traits are autosomal traits that are influenced by sex.
• If a male has one recessive allele, he will show that trait, but it will take two recessive for the female to show that same trait.
• •A sex-influenced trait is controlled by a pair of alleles found on the
autosomes. (Not on sex chromosomes)
Pattern Baldness is Sex-Influenced
The combination of alleles lead to different phenotypic expressions depending on gender.
Peacock Plumage is Sex-Influenced
The plumage of a male peacock is highly decorated and colored,
while the plumage of a female peahen is dull by comparison.
Sex-Linked Traits
If a gene is found only on the X chromosome and not the Y chromosome, it is said to be a sex-linked trait. Because the gene controlling the trait is located on the sex chromosome, sex linkage is linked to the gender of the individual. Usually such genes are found on the X chromosome.
When we talk about genes…
• Phenotype: What your physical appearance shows.
• Genotype: What your alleles show you.
What is DNA?
DNA stands for deoxyribonucleic acid and contains genetic
information.
• It is found on
chromosomes located
in
the nucleus of our cells.
What is DNA?• The sides or backbone of the DNA molecule are made up of sugar(deoxyribose) and phosphate molecules.• The rungs that form the middle of the molecule are made up of pairs ofnucleotides or nitrogen bases. Adenine (A) pairs with thymine (T), whileguanine (G) always pairs with cytosine (C).• The order of the bases determines the genetic code.
How is DNA used as Evidence• Each person’s DNA is different from other people (except identical twins).• DNA collected from a crime scene can either link a suspect to the evidence or eliminate asuspect, similar to the use of fingerprints.• DNA can identify a victim through DNA from relatives, even when no body can be found.• DNA can link crime scenes together by linking the same perpetrator to different sceneslocally, statewide, and across the nation.
How is DNA used as Evidence• DNA can place an individual at a crime scene, in a home, or in a room where thesuspect claimed not to have been.• DNA can refute a claim of self-defense and put a weapon in the suspect's hand.• It can change a story from an alibi to one of consent.
Factors that affect DNA
Several factors can affect the DNA left at a crime scene, such asenvironmental factors (e.g., heat, sunlight, moisture, bacteria, and mold).Therefore, not all DNA evidence will result in a usable DNA profile.Further, DNA testing cannot identify when the suspect was at the crimescene or for how long.
CODIS
CODIS stands for COmbined DNA Index System, which is an electronic database of DNA profiles that can identify suspects. DNA profiles from individuals convicted of certain crimes, such as rape, murder, and child abuse, are entered into CODIS and help officers identify possible suspects when no prior suspect existed.
Did you Know?
Each human cell contains three billion DNA base pairs. Our unique DNA
amounts to 0.1% or 3 million base pairs.
