Genes Introduction to Genetics

What is DNA?  DNA stands for DeoxyriboNucleic Acid  It is the hereditary material in humans and almost all other organisms.  Most DNA is located in the cell nucleus.  “Blueprint of life”  The main role of DNA molecules is the long-term storage of information. moleculesinformation moleculesinformation

DNA Structure  DNA is a double-stranded molecule.  The shape of the DNA molecule is a double-helix (think of a spiral staircase or like a twisted ladder).  The sides of the ladder are composed of alternating sugars (deoxyribose) and phosphates.  The rungs of the ladder are composed of nucleotides.

 The structuof DNA was established by James Watson and Francis Crick. DNA Structure

Base Pairing Rule Nucleotides (also called Bases) Adenine, Thymine,, Guanine, Cytosine or A, T, G, C Nucleotides pair in a specific way - called the Base-Pair Rule Adenine pairs to Thymine AT Guanine pairs to Cytosine GC Memory helper - Think "A T Granite City" - which is where you live.

What is a gene?  Gene - a segment of DNA that codes for a protein, which in turn codes for a trait (skin tone, eye color.etc),  a gene is a stretch of DNA.  Every person has two copies of each gene, one inherited from each parent.

What is a chromosome? Chromosomes are DNA molecule packaged into thread-like structures in the nucleus

How many chromosomes do people have?  In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs, called autosomes, look the same in both males and females.  Diploid a cell that contains “two sets” of chromosome 2N  Haploid a cell that contains “one set” of chromosome

Sex Determination  Sex cell chromosomes determine the sex of an organism  Sex cells are the sperm and egg– gametes  Females have two X chromosomes, (XX)  Males have one X and one Y chromosome (XY)  Zygote is the union of the sperm and egg (46 chromosomes).

 Heredity is the transmission of characteristics from parents to offspring.  The genes for many traits are passed down in families from parents to children.  Because you come from two parents, each parent will provide one half of your genes for any trait. That is called an "allele." So you have two alleles for each gene, one from each parent.

Gregor Mendel

 “Father” of genetics  Austrian monk, mid-1800s  Researched pea plant inheritance –Easy to grow, fast reproduction –Studied plant height, pea shape/color, pod color, etc.

Pea Plant Reproduction  Self-pollination –Male gametes (pollen) fertilize egg of same flower –Produces pure-bred offspring –True-breeding: produce offspring identical to parent when self-pollinated

Pea Plant Reproduction  Cross-pollination –Pollen from one plant fertilizes egg of another plant –Offspring have two parents

Mendel’s Experiments  P = Parent generation  F 1 = First filial generation  F 2 = Second filial generation (F 1 X F 1 ) –Outcome P Pure Green X Pure Yellow F 1 All Green F 2 3 Green:1 Yellow

Mendel’s Conclusions  Law of Dominance – one allele (form of a gene) is dominant, one is recessive –Recessive trait was hidden in F 1 generation –Green = dominant –Yellow = recessive

Mendel’s Conclusions  Law of Segregation: alleles for a gene separate when gametes form (meiosis I) –Each gamete gets one copy of each gene

Some Vocab.  Genotype – allele combination  Genotype is the genetic make up –Capital letter = dominant allele –Lowercase letter = recessive allele –Ex – AA, Aa, aa  Phenotype – physical appearance –Ex – green, yellow

Some Vocab.  Homozygous – two alleles Same  Homozygous organisms that have two identical alleles. –Homozygous dominant: AA –Homozygous recessive: aa  Heterozygous – two alleles different  Heterozygous organism with two different alleles. –Aa

Dominant and recessive  Dominant traits are the expressed characteristic. It is always represented with a capital letter  Recessive traits are the repressed alleles; the ones that are not expressed. It is always represented with a lower case

Probability  Punnett squares are used to predict the probability of certain traits in offspring of genetic crosses  Tt X Tt –½ chance of getting ‘t’ from mom, ½ chance of getting ‘t’ from dad –½ X ½ = ¼ tt in offspring

Punnett Squares  First must determine possible gametes  Heterozygous tall plant = Tt –Half of gametes will get ‘T’, other half will get ‘t’  Homozygous tall plant = TT –All gametes will get ‘T’

Punnett Squares  Monohybrid cross –Cross involving one trait  Gametes go on the top and side  Combine gametes to find possible offspring Tt X Tt

Punnett Squares  Genotype ratio 1TT: 2Tt: 1tt  Phenotype ratio 3 tall: 1 short Tt X Tt

Dihybrid Cross  Mendel looked at the inheritance patterns of two traits –Seed shape and seed color  Found that the traits were inherited independently of each other  Law of Independent Assortment –Genes on separate chromosomes are inherited at random –Due to random chromosome shuffling in Metaphase I

Independent Assortment Metaphase I

Non-Mendelian Genetics  Not all traits follow Mendel’s Law of Dominance  Four Variations –Incomplete Dominance (blending) –Codominance (two phenotypes) –Multiple Alleles –Polygenic Traits

Incomplete Dominance  Neither allele is dominant, both produce a protein  Heterozygous phenotype is a blend of both homozygous phenotypes  Ex – wavy hair, pink flowers

Incomplete Dominance

Codominance  Neither allele dominates the other, both produce a protein  Heterozygous phenotype is a combination of both homozygous phenotypes  Ex – checkered chicken, human blood types,

Multiple Alleles  Some genes have more than two alleles  Each individual only gets two, but there are more than two in the population  Ex – Rabbit fur color

Multiple Alleles  Rabbit fur alleles (in order of dominance) –C: dark gray –c ch : chinchilla –c h : himalayan –c: albino

Multiple Alleles

Polygenic Traits  Many traits are controlled by more than one gene  Traits show wide variation  Ex – human height, IQ, bell pepper colors

Sources  nt/begin/traits/activities/index.html nt/begin/traits/activities/index.html nt/begin/traits/activities/index.html