Human Genetics - Chapter 4

Human Genetics - Chapter 4

Chapter 4 Single-Gene Inheritance Learning Outcomes Describe how Mendel deduced that recessive traits seem to disappear in hybrids Define and distinguish heterozygote and

homozygote; dominant and recessive; phenotype and genotype Explain how the law of segregation reflects the events of meiosis Describe a Punnett square 2 Learning Outcomes (2)

Explain how a gene alone usually does not solely determine a trait Distinguish between autosomal recessive and autosomal dominant inheritance Explain how Mendels experiments followed the inheritance of more than one gene Explain how the law of independent assortment reflects the events of meiosis

3 Learning Outcomes (3) Explain how pedigrees show single-gene transmission Explain how exome sequencing in a family can reveal Mendelian inheritance patterns

4 Following the Inheritance of One Gene Modes of inheritance are the patterns in which single-gene traits and disorders occur in families Huntington disease is autosomal dominant

Affects both sexes and appears in every generation Cystic fibrosis is autosomal recessive Affects both sexes and can skip generations through carriers 5 Mendels Experiments

Described the units of inheritance and how they pass from generation to generation Mendel had no knowledge of DNA, cells, or chromosomes His laws of inheritance explain trait transmission in any diploid species Conducted experiments from 18571863 on

traits in 24,034 plants 6 Mendels Experiments (2) Deduced that consistent ratios of traits in the offspring indicated that plants transmitted distinct units Analyzed genetic crosses of peas

P1 - Parental generation F1 - First filial generation F2 - Second filial generation 7 Mendel Studied Transmission of Seven Traits in the Pea Plant

Mendels Experiments (3) True-breeding - Offspring have the same trait as parent Example - Short parents produce all short offspring

The observed trait is dominant The masked trait is recessive Monohybrid cross follows one trait Self-crossed plants are hybrids 9

Figure 4.2 Monohybrid Cross Experiments confirmed that hybrids hide one expression of a trait, which reappears when hybrids are self-crossed Mendel speculated that each elementen

was packaged in a separate gamete Law of segregation is Mendels idea that elementen separate in the gametes 11 Mendels First Law - Segregation Reflects the actions of chromosomes and the

genes they carry during meiosis Homozygous carry same alleles TT or tt Heterozygous carry different alleles Tt Genotype = Organisms alleles Phenotype = Outward expression of an allele combination Wild Type = Most common phenotype

Recessive or dominant 12 Mendels First Law Segregation (2) Mutant phenotype = Variant of a genes expression that arises when the gene undergoes mutation Mendel observed the events of meiosis

Two copies of a gene separate with the homologs that carry them when a gamete is produced At fertilization, gametes combine at random 13 Mendels First Law Segregation (3)

Mendels Data Punnett Square Represents how genes in gametes join if they are on different chromosomes Test Cross A monohybrid cross yields:

A 1 TT : 2 Tt : 1 tt genotypic ratio, and A 3 tall : 1 short phenotypic ratio Mendel distinguished the TT from Tt tall plants with a test-cross Cross an individual of unknown genotype with a homozygous recessive individual

17 Test Cross Inheritance of Some Common Traits Single-Gene Inheritance Single-gene disorders are rare

Phenotypes associated with single genes are influenced by other genes and environmental factors 20 Eye Color People differ in the amount of melanin and

number of melanosomes Have the same number of melanocytes The surface of the back of the iris contributes to the intensity of eye color OCA2 confers eye color by controlling melanin synthesis HERC2 controls expression of the OCA2 gene

21 Eye color (2) Modes of Inheritance Rules that explain the common patterns of single-gene transmission Passing of a trait depends on whether:

Determining gene is on an autosome or on a sex chromosome Allele is recessive or dominant Autosomal inheritance can be dominant or recessive 23

Autosomal Dominant Traits 24 Autosomal dominant inheritance Criteria for Autosomal Recessive Traits Males and females can be affected

Affected males and females can transmit the gene, unless it causes death before reproductive age Trait can skip generations Parents of an affected individual are heterozygous or have the trait Conditions likely to occur in families with consanguinity

26 Solving Genetic Problems Follow these five general steps: List all genotypes and phenotypes for the trait Determine the genotypes of the parents Derive possible alleles in gametes Unite gametes in all combinations to reveal all

possible genotypes Repeat for successive generations 27 On the Meaning of Dominance and Recessiveness Knowing whether an allele is dominant or

recessive is important in determining risk inheriting a particular condition Reflect the characteristics or abundance of a protein Recessive traits are due to loss of function Recessive disorders tend to be severe, produce symptoms earlier than dominant disorders

Dominant traits arise from gain of function 28 Loss or Gain of a Function Mendels Second Law - Independent Assortment

Considers two genes on different chromosomes The inheritance of one does not influence the chance of inheriting the other Two genes that are far apart on the same chromosome appear to independently assort Numerous crossovers take place between them

30 Mendel's Second LawIndependent Assortment (2) Plotting a Dihybrid Cross Probability The likelihood that an event will occur

Product rule - Probability of simultaneous independent events equals the product of their individual probabilities Predicts the chance of parents with known genotypes to produce offspring of a particular genotype Example - Consider the probability of obtaining a plant with wrinkled, green peas (genotype rryy ) from dihybrid

( RrYy ) parents 33 Product Rule Do the reasoning for one gene at a time, then multiply the

results Using Probability to Track Three Traits Pedigree Analysis For researchers, families are tools; the bigger the family, the easier it is to discern modes of inheritance

Pedigrees are symbolic representations of family relationships and the transmission of inherited traits 36 Pedigree Analysis (2)

An Unusual Pedigree A partial pedigree of Egypts Ptolemy Dynasty showing: Genealogy not traits Extensive

inbreeding Pedigree - Marriage of First Cousins Importance of Pedigrees Today Helps families identify the risk of transmitting an inherited illness Starting points for identifying and describing,

or annotating, a gene from the human genome sequence Meticulous family records are helping researchers follow the inheritance of particular genes 40 Autosomal Recessive Trait

Albinism = Deficiency in melanin production Parents are inferred to be heterozygotes Autosomal Dominant Trait Does not skip generations, can affect both sexes Polydactyly = Extra fingers and/or toes

42 An Inconclusive Pedigree This pedigree can account for either an autosomal dominant or an autosomal recessive

trait Passed in an autosomal dominant mode Conditional Probability Pedigrees and Punnett squares apply Mendels laws to predict the recurrence risks of inherited conditions

Example: Taneeshas brother Deshawn has sickle cell disease What is the probability that Taneeshas child inherits her mutant allele and be a carrier? 44 Making predictions

Probability Taneesha is a carrier = 2/3 Probability child inherits sickle cell allele = 1/2 Probability child carries sickle cell allele from her = 2/3 x 1/2 = 1/3 Taneesha is not affected and cannot be ss Taneesha and Deshawns parents must be heterozygous

Family Exome Analysis Comparing DNA sequence of the exome of a relative with unexplained symptoms or traits to the exomes of other family members Useful in identifying a disease-causing gene variant inherited from a parent, or one that has arisen in the child

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