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2/25/02 Lecture Highlights: Inheritance Inheritance - Outline • Human phenotypes • Genotypes (dominant and recessive) • How are they inherited? – Mendel’s law of segregation – Mendel’s law of independent assortment Can you roll your tongue? Clasp your hands together, interlacing your fingers… Which thumb is on top- left or right? Widow’s peak – dominant trait Detached earlobes – dominant trait Attached earlobes – recessive trait • • Leonardo DiCaprio and Parker Posey’s phenotypes are detached earlobes/ widow’s peak Matt Damon’s phenotype is attached earlobes/ widow’s peak • • So, phenotype is a description of physical appearance Genotype is the genetic basis for appearance (and other unseen traits) What do dominant and recessive mean? First we need to look at a cell…. • Chromosomes are found in the nucleus • Human chromosomes are in pairs (we are diploid- see glossary) • One pair of chromosomes – homologous • locus – location of a particular gene on a chromosome • allele – different forms of a gene at a locus • homozygous - 2 copies, same allele (e.g., blue/blue) • heterozygous -2 copies, diff. alleles (e.g., red/green) Let’s say red allele means detached earlobes and green allele means attached 9 Remember… attached earlobes are dominant trait 9 So, the person’s genotype is red/green, BUT – their phenotype is detached earlobes green is masked! This locus codes for hairline (widow’s peak = dominant; straight hairline = recessive) 9 Now let’s say the blue genotype means straight hairline phenotype 9 This person will have straight hairline (results only when both alleles are blue) • dominant allele –one dominant allele present in the genotype = automatically expressed in the phenotype (masks recessive) • recessive allele – both copies on homologous chromosomes must be same for recessive to be expressed in phenotype Inheritance We get 1 set of chromosomes from mom & 1 from dad • I can roll my tongue (dominant) but my mom can’t (recessive) – • let’s say now R is the allele for rolling and r is allele for not • Can my dad roll his tongue? • • What’s my mom’s genotype? What’s my dad’s genotype? • Can my sister tongue roll? Let’s say dad is heterozygous…(Punnett square) Dad’s sperm R r Mom’s eggs r r Rr Rr rr rr Possible offspring genotypes (bold) Bird breeding example… Gregor Mendel (1822-1884) • Augustinian monk (Austria) • Studies on plant hybridization/ inheritance • Laid foundation for modern genetics • Results published in 1865 – BUT ignored until 1900 Mendel’s first experiment – pea plants with purple and white flowers 1. Removed stamens from purple flower (prevent self-fertilization) 2. Transferred pollen from white to purple flower 3. plants made seeds 4. planted seeds and observed offspring (F1) phenotypes 5. Bred F1s together 6. observed F2 (“grandchild”) phenotypes • • Results: F1 generation all purple; F2 generation ¾ purple, ¼ white (purple dominant, white recessive) These experiments resulted in Mendel’s principle of segregation – pairs of alleles segregate (separate) during gamete formation (when eggs and sperm are made); fusion of gametes at fertilization creates allele pairs again Mendel looked at many phenotypic traits in pea plants • Through other crosses- determined yellow dominant to green and round peas dominant to wrinkled Mendel’s second experiment (dihybrid cross; 2 traits – pea color and pea shape) • F1 – all yellow, round (dominant traits) • F2 – not 3:1 ratio, but 9:3:3:1 ratio o He knew something other than segregation was going on…. • Principle of independent assortment o Each pair of alleles segregates independently of one another during gamete formation Glossary of genetics terms: genotype – genetic type (what genes a person has for a particular trait; usually represented by 2 alleles; for example genotype Ff) phenotype – physical appearance (influenced by one or many genes; for example a person with genotype Ff or ff has freckles, which is their phenotype) gene – segment of DNA that codes for a particular protein allele – different forms of a gene chromosome – tightly wound up DNA that contains many loci (and therefore many genes) homologous chromosomes – pairs of chromosomes that contain the same genes (inherit one from mom and one from dad) diploid – 2 copies of chromosomes (and hence the genes on them) homozygous – 2 copies of the same allele (e.g., FF or ff) on homologous chromosomes heterozygous – different alleles on homologous chromosomes (e.g., Ff) Punnett square –determines offspring genotypes by matching up parental genotypes Genetics Concepts dominant – if one dominant allele is present in the genotype, it is automatically expressed in the phenotype although there are 2 copies in diploid organisms; it masks the other allele on the other chromosome); Freckles are a dominant trait, so if a person has either genotype FF or Ff, they will have the freckle phenotype. recessive – for a recessive allele to be expressed, it has to be present on both chromosomes. No freckles are recessive, so a person must have the genotype ff to have no freckles as their phenotype. codominant – both copies are expressed (e.g., “red” gene on one chromosome and “white” gene on another chromosome -> genotype = red/white; but phenotype = pink). A plant with codominant flower color genes would have red flowers if the genotype is RR, white flowers if the genotype is WW and pink flowers if the genotype is RW