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GENETICS Genetics • The study of heredity-passing on characteristics from parents to offspring. • Gregor Mendel (1860’s) discovered the fundamental principles of genetics by breeding garden peas. “Father of Genetics” • Gene – segment of chromosome (DNA) that codes for a trait • Trait -specific characteristics – Green pea color Generation lingo •P1 generation - parents, first initial cross •F1 generation - first offspring resulting from crossing the P1 generation •F2 generation - second set of offspring that resulted from crossing the F1 generation Mendel’s Factors 1. Units that determine heritable traits. 2. Versions of gene = Alleles - Two alleles for every trait - One allele from each parent - Dominant and Recessive Letters used to represent alleles. Capital letter = Dominant ex: B Lowercase letter = recessive ex: b Complete (simple) Dominance • Dominant = masks/covers up the other allele for a specific characteristic. • Recessive = can be “masked” by dominant. Will only see recessive trait if dominant allele is absent. Genetic Terms Homozygous – both alleles are the SAME type of allele. Homozygous Dominant – both alleles are dominant. Will show the dominant trait. -TT Homozygous Recessive – both alleles are recessive. Will show the recessive trait. -tt Heterozygous – two DIFFERENT alleles. One dominant and one recessive. Will show the dominant trait. -Tt Genotype • Version of gene (alleles) inherited from parents (one allele from each) . • Example: TT tt Tt Phenotype • Outward appearance • Physical characteristics • Examples: tall pea plant Freckles Brown fur Long tail Baby Bunny Activity Mendel’s findings • Law of Segregation =alleles are separated during the formation of gametes. – ONE allele from EACH PARENT • Law of Independent Assortment = alleles for different traits are distributed randomly to gametes independently. • These only occur in sexual reproduction – Produce genetic variety – Genetic recombination! Law of Segregation Meiosis - eye color B sperm B B Bb haploid (n) b diploid (2n) b b meiosis I meiosis II Homologous Chromosomes eye color locus B = brown eyes eye color locus b = blue eyes Paternal Maternal Law of independent assortment – the eye color location (locus) of chromosome is not the same as another trait, say hair color. Punnett square • A Punnett square is used to cross two parents and show the possible offspring combinations of gametes. Monohybrid Cross • A breeding experiment that tracks the inheritance of a single trait. • Types of monohybrid cross – Homozygous x homozygous – Homozygous x heterozygous – Heterozygous x heterozygous Breed the P generation • tall (TT) vs. dwarf (tt) pea plants T t t T tall (TT) vs. dwarf (tt) pea plants T T t Tt Tt produces the F1 generation t Tt Tt All Tt = tall (heterozygous tall) 100% tall – phenotype % 100% Tt – genotype % Breed the F1 generation • tall (Tt) vs. tall (Tt) pea plants T T t t tall (Tt) vs. tall (Tt) pea plants T T t TT Tt t Tt tt produces the F2 generation 1/4 (25%) = TT (Tall) 1/2 (50%) = Tt (Tall) 1/4 (25%) = tt (short) 1:2:1 genotype ratio 3:1 phenotype ratio More terms to know… • Purebred – PURE = SAME = Homozygous – HH – hh • Hybrid – Different = Heterozygous – Hh Review Monohybrid Cross • Example: Cross between two heterozygotes for brown eyes (Bb) BB = brown eyes Bb = brown eyes bb = blue eyes B b B Bb x Bb b female gametes male gametes Monohybrid Cross B b B BB Bb b Bb bb Bb x Bb 1/4 or 25% = BB - brown eyed 1/2 or 50% = Bb - brown eyed 1/4 or 25% = bb1:2:1 - blue eyed ratio genotype 3:1 phenotype ratio Sex-linked Traits • Traits (genes) located on the sex chromosomes • XX = girl • Xy = boy • The y chromosome is smaller than the X chromosome and is lacking corresponding alleles. Karyotype The 23rd pair is the sex chromosomes. Should be able to determine gender. Notice the Y chromosome size in comparison to the X. Sex-linked traits • Will find predominantly (higher % occurrence) in MALES. • Males can not be heterozygous (carriers) for a sex-linked trait. • They either have it or not…only get one allele on the X chromosome. Queen Victoria of England was a carrier of the gene for hemophilia. She passed the harmful allele for this X-linked trait on to one of her four sons and at least two of her five daughters. Her son Leopold had the disease and died at age 30, while her daughters were only carriers. As a result of marrying into other European royal families, the princesses Alice and Beatrice spread hemophilia to Russia, Germany, and Spain. By the early 20th century, ten of Victoria's descendants had hemophilia. All of them were men, as expected. Queen Victoria (1819-1901) with her husband and nine children in 1857 Sex-linked Traits • Example: fruit flies (red-eyed male) X (white-eyed female) • Remember: the Y chromosome in males does not carry traits. RR = red eyed Rr = red eyed rr = white eyed Xy = male XX = female XR Xr Xr y Sex-linked Traits XR Xr XR Xr y Xr y 1/2 red eyed and female 1/2 white eyed and male Xr XR Xr Xr y Incomplete Dominance • F1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties. • With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. R R Example: snapdragons (flower) r red (RR) x white (rr) RR = red flower r rr = white flower Incomplete Dominance R R r Rr Rr produces the F1 generation r Rr Rr All Rr = pink (heterozygous pink) Both alleles expressed as “mix” Codominance • the prefix "co-" is "together" • Two alleles are expressed (multiple alleles) in heterozygous individuals. • Neither of two alleles masks the other • In COdominance, both traits appear together equally (at the same time) in the phenotype of hybrid organisms. Codominance • Example: blood 1. 2. 3. 4. type A type B type AB type O = = = = IAIA or IAi IBIB or IBi IAIB ii Examples • A very common phenotype used in questions about codominance is roan fur in cattle. Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together). A good example of codominance. Mutations • Mistakes – errors – changes in the DNA • Can happen during DNA replication and Protein Synthesis (transcription) • How? – DNA breaks apart (bonds broken) – Nitrogen bases are paired incorrectly – Nitrogen bases switch spots with neighboring bases = paired incorrectly – Nitrogen bases deleted Why do mutations occur? • Random • Genetically inherited from parental gametes – in alleles • Caused by mutagens. What are mutagens?....(wait for it…) Mutagens • Environmental factors that cause mutations – DNA molecule to fall apart – bonds are broken • High heat & temperature – Nitrogen base changes • • • • UV radiation (sun) Nuclear radiation Chemicals – such as formaldehyde Some viruses Are all mutations bad? • Some mutations can result in providing an organism with an advantage for survival. – New variations in species – Dumbo…bigger than normal ears. – What if big ears give an advantage? Applying what you know… • DNA RNA (amino acid) Protein Trait • If you wanted to find out if two organisms were more closely related, how would you do that? – What could you analyze? DNA Sequence (order of nitrogen bases) Which of the two organisms are most closely related? • • • • ATTCCGGCAATG ATTCCCCCAATG ATTTAACGTAAC ATAGGAATACGG Amino Acid Sequence Traits • Remember when you sorted the organism pictures based on what you could see (their traits). • Outward traits (phenotypes) are determined by genotype. • More similar traits then more likely to have more similar DNA