Bio.5c Genetics 24-25-7 PDF

Bio.5c – Genetics & Heredity BIO.5c – We are learning how traits are passed down from parents to offspring.  I can describe the relationship between genotype, phenotype, allele, and genes  I can use monohybrid and dihybrid Punnett squares to predict the expected ratios of genotypes and p...

Bio.5c – Genetics & Heredity BIO.5c – We are learning how traits are passed down from parents to offspring.  I can describe the relationship between genotype, phenotype, allele, and genes  I can use monohybrid and dihybrid Punnett squares to predict the expected ratios of genotypes and phenotypes in offspring  I can compare inheritance patterns for dominant, co-dominant and incompletely dominant traits  I can interpret a pedigree to determine the sex, genotype, and phenotype of an individual GENETICS ….the science of how genes are inherited on chromosomes from one generation to the next. Genes & Chromosomes  Chromosomes are DNA molecules  Genes are segments of DNA on chromosomes that code for traits  Traits are the characteristics that an organism has like: Hair color Eye color Height Genetic variation  Genes code for traits like eye color and hair color  Alleles are different versions of a gene that produce different variations of a trait, like brown or blue eye color  Traits are determined by the alleles we inherit from our parents Mutation is the source of variation  A mutation is a change in the DNA sequence  New alleles arise by mutation and can help an organism survive or can cause disease Mendelian Genetics Mendel was an Austrian Monk and scientist. He demonstrated that traits are inherited in predictable patterns. These patterns can be explained by the:  Law of Dominance  Law of Segregation  Law of Independent Assortment Mendel’s Experiments  He focused on seven traits found in pea plants.  Each trait had two variants  Tall vs Dwarf  Round vs Wrinkled  Yellow vs Green  P generation = Parental generation  F1 Generation = First generation of offspring  F2 Generation = Second generation of offspring  Hybrids result form the cross of true-breeding parents Cross-breeding plants  Mendel crossed tall plants with dwarf plants and all the F1 offspring were all tall.  The trait of the other parent seemed to have disappeared.  Mendel thought the dwarf trait had been lost. Tall plants x dwarf plants → Produces only tall F1 generation F1 generation  Hybrid (heterozygotes) Cross  Next, Mendel allowed the hybrid F1 offspring to self- pollinate to see what traits the F2 generation carried.  The recessive trait had reappeared! F1 Tall x F1 Tall → Produces 3 tall : 1 dwarf F2 generation How can these probabilities be explained? Law of Dominance  Why do some traits ‘disappear’?  Some alleles are dominant:  Only 1 copy needed for the trait to be expressed (visible)  Some alleles are recessive:  2 copies are needed for the trait to be expressed (visible)  It is masked by the presence of the dominant allele and so it ‘disappears’! Law of Dominance  The tall allele (T) is dominant over the dwarf allele (t)  A diploid organism has two alleles for every gene  The dominant allele masks the recessive allele in the heterozygote Genotype Phenotype TT Tall plant Tt Tall plant tt Dwarf plant Genotypes and Phenotypes  Genotype is the genetic makeup of an organism (two alleles: TT, Tt or tt)  Phenotype is the physical characteristics or traits we can observe (like flower color or plant height) Phenotypes: One flower is purple One flower is pink Three plants are tall One plant is short Genotypes Genotypes Homozygous vs Heterozygous  A genotype is homozygous is when both alleles are the same  A genotype is heterozygous is when the two alleles are different Homozygous Homozygous Dominant Recessive Law of Segregation  Every diploid organism carries two alleles for each trait T = dominant allele for Tall t = recessive allele for short  During meiosis, alleles separate (aka segregate) to form gametes (eggs/sperm)  Each parent passes one allele of a gene to their offspring (not both)  Gametes are randomly combined during fertilization  This creates diversity in the gametes and in the offspring after fertilization Probability  This is a diagram that allows you to calculate the probability of different offspring based on the genotypes of the parents.  Probability is the likelihood that a particular event will occur  Capital letters = dominant  Lowercase letters = recessive Monohybrid Cross  When only one trait is being considered, this is monohybrid cross A a  In this cross, the female parent is heterozygous (purple) and the male parent is homozygous recessive (pink) a  What is the predicted ratio of genotypes in the offspring? What is the predicted ratio of  phenotypes in the offspring? a Practice  Normal skin color is dominant over albino skin (A, a)  Skin color is controlled by the melanin pigment protein.  A woman is albino, she does not make melanin.  She marries a heterozygous, normal skinned man.  What type of offspring might they expect? Law of Independent Assortment  Different alleles of different genes have an equal opportunity to occur together.  This is assuming that the two genes are located on separate chromosomes.  So, does the gene that determines pea plant size (tall vs dwarf) have any effect on the gene for seed color (green vs yellow)? No, these alleles assort independently! Law of Independent Assortment  Each allele can segregate (separate) and combine randomly Gamete possibilities if you have genotype: RrYy Dihybrid (Two-factor) Cross  This is when two traits are determined YYRR  This Punnett Square has 16 boxes!  Each gene/allele is given a letter.  Yellow (Y) vs Green (y)  Round (R) vs Wrinkled (r)  YYRR, yellow and round  yyrr, green and wrinkled  What is the phenotype of YyRr? Practice Reading a Dihybrid Cross! Hybrid cross: YyRr x YyRr YYRR How many will be yellow and round? 9/16 How many will be yellow and wrinkled? 3/16 How many will be green and round? 3/16 How many will be green 1/16 and wrinkled? Why is it more likely offspring will be yellow and round? Incomplete Dominance  Offspring have a phenotype that is a intermediate between the parental phenotypes.  Alleles are neither dominant nor recessive  Alleles blend together so there are no recessive alleles  Use only capital letters  Red = R (or CR)  White = W (or CW)  Dominant traits are red and white  Incompletely dominant trait is pink Polygenic Inheritance  Many traits are controlled by multiple genes with more than two alleles.  Skin color has 10 identified genes which have different alleles and mutations  Eye color has 15 different genes with their own set of alleles Codominance  Two dominant alleles are fully expressed at the same time. This creates a third phenotype.  Alleles DO NOT blend together in the offspring  Both alleles are visible in the phenotype of the offspring Erminette chickens: White (WW) Black (BB) Erminette (BW) Codominance & Multiple Alleles  Humans have four blood types: A, B, AB, and O  A and B alleles are codominant  o is a recessive allele Genotype Phenotyp  Presence of the Rh factor dominant and the e absence is recessive (separate gene) IAIA AA Type A IAi Ao Type A IBIB BB Type B IBi Bo Type B IAIB AB Type AB ii oo Type O Blood Typing Practice  What are the genetic possibilities when a heterozygous Type A man has children with a heterozygous Type B woman?  Genotype man =  Genotype woman = What if genes are carried on sex chromosomes? (extension) Some genes are located on the X chromosome. In the Punnett square, we must include the sex chromosomes Notice that males only have one copy of a sex-linked allele In this cross, only 25% of offspring are affected, but these are all male 50% of females are heterozygous Color-blindness is a X-linked trait Interpreting a Dihybrid Cross 1. What proportion of the offspring are tan with red eyes? (both dominant traits) 2. What proportion of the offspring are black with tan eyes? (both recessive traits) 3. What are the genotypes of the parents? CER Practice A student crosses two tall pea plant with purple flowers. They believe that tall and purple are the dominant traits. They work in a team to collect data and record the traits for 537 peas. They make the claim that the parents were both hybrids (heterozygous) based on the evidence below. Can you justify their claim?? Observed Expected Justification: Tall with purple flowers 310 9/16 x ___ = ____ Tall with white flowers 98 3/16 x ___ = ____ Short with purple flowers 101 3/16 x ___ = ____ Short with white flowers 28 1/16 x ___ = ____ Total 537 What are Pedigrees?  Allows us to trace inheritance in family trees  Often used to observe inheritance of a genetic disease  Circles = females  Squares = males  Shaded are generally affected with the disease/disorder  What is a ‘carrier’?  Someone who is heterozygous for a recessive disease-causing allele  Carriers are not affected, but can pass the disease- causing allele to their offspring  If both parents are carriers, what is the probability their child will be affected with the disease? Interpreting a Pedigree  The inheritance of a trait – dominant or recessive; autosomal or sex-linked  We can predict the genotype of individuals in the pedigree  Start with patterns:  Does the trait skip generations? If so, it’s a recessive trait!  If not, it could be dominant Interpreting a Pedigree  How many generations are shown  How do we know this is a recessive trait? If this is an autosomal, recessive trait:  What is the genotype of individual II-3?  What are the genotypes of individuals III-1 and III-2? A researcher hypothesizes that this may be a sex-linked trait. What evidence supports this hypothesis?