Observing Patterns in Inherited Traits

Observing Patterns in Inherited Traits
Chapter 8

Terms and Concepts Gene Locus
Heritable unit of information about traits One gene generally codes for one protein In a diploid cell there are pairs of genes One of the pair on each of the homologous chromosomes Locus Location of a gene on the chromosome

Terms and Concepts Allele
Different molecular forms or traits of the same gene Arise by mutation A permanent change in a gene and in the information it carries Diploid cells have two alleles for each gene

Terms and Concepts Types of alleles Dominant Recessive
Its trait is always expressed Masks the effect of a recessive allele Represented with a capital letter in inheritance problems (A) Recessive Expressed only when paired with another identical recessive allele Its trait is masked by dominant alleles Represented with a lower case letter in inheritance problems (a)

Terms and Concepts Combinations of alleles Homozygous condition
Homologs carry the same allele A pair of identical alleles belong to a true-breeding lineage Individuals can be either homozygous recessive (aa) or homozygous dominant (AA) Heterozygous condition Homologs carry different alleles Individuals are referred to as heterozygous (Aa) or hybrids (result of a cross between two different true-breeding individuals)

Terms and Concepts Gene expression
Process by which a gene’s information is converted to a structural or functional part of a cell Transcription of DNA to mRNA Translation of RNA to protein Determines traits

Terms and Concepts Genotype Phenotype
Particular alleles that an individual carries Examples: AA, Aa, aa Phenotype Refers to an individual’s traits Examples: color, shape, size, texture, etc.

Terms and Concepts Genetic crosses
Two individuals are crossed and the resulting offspring are examined to determine inheritance patterns P stands for the parents F1 stands for the first-generation offspring of crossed P individuals F2 stands for the second-generation offspring of intercrossed F1 individuals

Questions Gene An individual’s traits Locus Heterozygous
Dominant allele Second generation offspring True-breeding Homozygous Hybrids An individual’s genes Genotype A trait that is always expressed Phenotype Heritable unit of information F2 generation Location of a gene

Genetic Crosses The following slides will present a genetic cross demonstrating the use of the above terminology and the use of punnett squares

Genetic Crosses Whether a person has attached or detached earlobes depends on a single gene with two alleles (We can name the gene with a letter “e”) Dominant allele is detached ear lobes Referred to as E (capital for dominant) Recessive allele is attached ear lobes Referred to as e (lower case for recessive)

Genetic Crosses Each individual inherits one allele from each parent
Depending on what combination of alleles are inherited will determine the genotype and phenotype of the individual Inherit two dominant alleles Genotype = EE or homozygous dominant Phenotype = detached earlobes Inherit two recessive alleles Genotype = ee or homozygous recessive Phenotype = attached earlobes Inherit one dominant allele and one recessive allele Genotoype = Ee or heterozygous The dominant allele will always mask the recessive allele’s trait

Genetic Crosses Punnett squares can be used to determine the probability of the genotypes and phenotypes of offspring of any given cross such as the following If we crossed a homozygous dominant dad with a homozygous recessive mom, what would the offspring genotype(s) and phenotype(s) be?

Genetic Crosses First you need to determine the genotype of the parents Dad = homozygous dominant = EE Mom = homozygous recessive = ee

Genetic Crosses Second, determine what each parent’s gametes will be
Based on what we know about meiosis we can determine what allele the gametes will carry (see figure 10.5) Remember that during meiosis homologous pairs are separated (anaphase I). One of the two alleles is on one of the homologs, the other is on the other homolog. Therefore, during meiosis one “E” will segregate into one gamete, while the other “E” will segregate into the other gamete Dad’s gametes will be E and E Mom’s gametes will be e and e

Genetic Crosses Third, place the gametes in a punnett square e e E
Dad’s go vertically in the first column Mom’s go horizontally across the top e e E

Genetic Crosses Fourth, determine what the possible outcomes are if either of dad’s gametes fuses with either of mom’s eggs e e E Ee Ee

Genetic Crosses Fifth, determine the probability of the genotypes and phenotypes Genotype possibilities are EE, Ee, or ee Count up how many out of four of each combination are in the punnett square EE : Ee : ee 0 : 4 : 0 Answer

Genetic Crosses Fifth, determine the probability of the genotypes and phenotypes Phenotype possibilities are Detached or Attached Count up how many out of the four of each trait are in the punnett square Detached : Attached 4 : 0 Answer

Genetic Crosses Punnett Square Practice
Cross a heterozygous dad with a homozygous dominant mom Ee X EE Cross a heterozygous dad with a heterozygous mom Ee X Ee Cross a homozygous recessive dad with a heterozygous mom ee x Ee

Gregor Mendel Using pea plants Gregor Mendel determined inheritance patterns Pea plants are self-fertilizing and so develop “true-breeding” varieties (homozygous) Mendel could open a floral bud of a true-breeding plant and snip out its anthers (contains pollen grains). The buds can then be brushed with pollen from a different true-breeding plant. Following observable differences between plants Mendel predicted that he would be able to follow certain traits and see if there were patterns in its inheritance.

Fig. 10-3, p.154

Gregor Mendel Theory of Segregation
Diploid cells have pairs of genes, on pairs of homologous chromosomes The two genes of each pair are separated from each other during meiosis, so they end up in different gametes Mendel used monohybrid crosses to demonstrate segregation

Gregor Mendel: Monohybrid Cross
Pea flower color Cross 1 True-breeding purple flowering plants were crossed with true-breeding white flowering plants (these are the parental generation, P) The offspring or F1 generation were all purple flowering Cross 2 The F1 generation were allowed to self-fertilize The offspring or F2 generation had a ratio of 3 purple flowering plants to 1 white flowering plant

Pea flower color Mendel was able to infer that Both parents must have two “units” of information Each parent transferred one of their “units” of information to the offspring The purple color dominated the white color The recessive white color shows up in ¼ of the F2 generation

Homozygous dominant parent Homozygous recessive parent
(chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring Fig. 10-5, p.156

Pea flower color Purple is dominant = A White is recessive = a Genotypes True-breeding purple genotype = AA True-breeding white genotype = aa Punnett square for cross 1 a a A Aa Aa

Fig. 10-7b, p.157

Pea flower color F1 are allowed to self-fertilize Punnett square for cross 2 A a A AA Aa a Aa aa

Fig. 10-7c, p.157

Fig. 10-6, p.156

Gregor Mendel Test cross A method of determining genotype
To determine the genotype of the F1 purple-flowering plants (could be AA or Aa) Mendel could cross them with true-breeding white-flowered plants (aa) If the F1 is AA, then all of the flowers would be purple If the F1 is Aa, then half of the flowers would be purple and half white Try the crosses on a punnett square

Gregor Mendel Theory of Independent Assortment
As meiosis ends, genes on pairs of homologous chromosomes have been sorted out for distribution into one gamete or another, independently of gene pairs on other chromosomes This is due to random alignment during meiosis

Gregor Mendel Theory of Independent Assortment
Mendel used dihybrid crosses to explain how two pairs of genes are sorted into gametes independently The following slides will demonstrate the type of dihybrid crosses used

Gregor Mendel: Dihybrid Cross
Pea flower color AND plant height Cross 1 True-breeding purple flowering tall plants were crossed with true-breeding white flowering dwarf plants (these are the parental generation, P) The offspring of F1 generation were all purple flowering tall Mendel’s question was whether purple flowering would always be linked to tall or whether purple could go with dwarf and white with tall. Looking at the F2 generation from cross 2 answered his question.

One of two possible alignments The only other possible alignment
a Chromosome alignments at metaphase I: A a a A A a a B B b b b b B B b The resulting alignments at metaphase II: A A a a A A a a B B b b b b B B B A A B b a a b b A A b B a a B c Possible combinations of alleles in gametes: AB ab Ab aB Fig. 10-8, p.158

Pea flower color AND plant height Cross 2 The F1 generation were allowed to self-fertilize The offspring or F2 generation had a ratio of 9 purple flowering tall plants 3 purple flowering dwarf plants 3 white flowering tall plants 1 white flowering dwarf plant

Pea flower color AND plant height Mendel was able to infer that Purple was not linked to tall and white was not linked to dwarf The two different genes did in fact sort independently

Pea flower color AND plant height Purple = A and white = a Tall = B and dwarf = b Genotypes True-breeding purple tall genotype = AABB True-breeding white dwarf genotype = aabb Punnett square for cross 1 ab ab AB AaBb AaBb

Pea flower color AND height F1 are allowed to self-fertilize Possible gametes for AaBb are AB, Ab, aB, ab

Pea flower color AND plant height Punnett square for cross 2 AB Ab aB ab AB AABB AABb AaBB AaBb Ab AABb AAbb AaBb Aabb aB AaBB AaBb aaBB aaBb ab AaBb Aabb aaBb aabb

Fig. 10-9, p.159

Questions T or F: The Theory of Segregation states that the two genes of each pair stay together during meiosis What type of cross was used to show Mendel’s Theory of Segregation? What is a Punnett Square? What genotype and phenotype ratios are seen in the F2 generation? T or F: The Theory of Independent Assortment states that gene pairs sort independently What type of cross was used to show Mendel’s Theory of Independent Assortment?

Beyond Simple Dominance
Mendel studied traits that have clear cut dominant and recessive forms Some genes can have alleles that are codominant or incompletely dominant

Codominance Non-identical alleles are both fully expressed even in heterozygotes Blood type IA and IB alleles are both dominant They are always expressed i is recessive Genotype Phenotype IAIA and IAi Type A blood IBIB and IBi Type B blood IAIB Type AB blood (codominant, they are both expressed) ii Type O blood

Fig , p.160

Incomplete dominance One allele isn’t fully dominant over the other allele, so the heterozygote’s phenotype is somewhere between the two homozygotes Snapdragon flower color Red flowers = RR White flowers = rr Heterozygotes , Rr are pink

Fig , p.160

Epistasis Some traits are the results of interactions of two or more gene pairs Labrador coat color Gene encoding pigment black is dominant to brown Gene encoding deposition of pigment Dominant allele promotes deposition of pigment Recessive allele reduces deposition The two genes work together to determine how much of what color pigment ends up in the coat

EB Eb eB eb EEBB black EEBb black EeBB black EeBb black EB EEBb black
chocolate EeBb black Eebb chocolate Eb EeBB black EeBb black eeBB yellow eeBb yellow eB eeBb yellow eebb yellow EeBb black Eebb chocolate eb Fig , p.161

Fig , p.161

Pleiotropy One gene can influence two or more traits Marfan syndrome A mutated form of the fibrillin gene affects the formation of connective tissues, thus its affects are seen in several areas of the body

Linkage Groups Some alleles tend to be inherited as a group
Mendel’s theory of independent assortment only works for genes located on different chromosomes If genes are located on the same chromosome, then they are generally linked In some cases crossing over during meiosis will separate linked genes depending primarily on how close the two genes are on the chromosome

p.162b

Fig , p.162

Genes and the Environment
Some genes and can be influenced by the environment Temperature affects coat color on Himalayan rabbits Cooler body parts are dark while the main body mass is warmer and creating a lighter coat color

Fig , p.163

Complex Variations in Traits
Individuals of populations can show continuous variation in a trait if there are multiple genes and environmental factors that influence a trait Height Eye color Skin color

Fig a, p.164

Fig b, p.164

Fig c, p.164

Summary Terms and Concepts Genetic Crosses Mendel
Segregation Independent Assortment Beyond simple dominance and other variations of inheritance patterns

Observing Patterns in Inherited Traits

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