Genetics: Study Of The Mechanisms Of Heredity Mendelian Genetics - - PowerPoint PPT Presentation

Genetics: Study Of The Mechanisms Of Heredity

• Mendelian Genetics proposed in mid-1800s by Gregor Mendel
• determined that inherited characteristics consisted of alleles that are

dominant or not

• Subsequent discoveries like the material of heredity (DNA) have added to

Mendel’s work but not completely replaced it!

• DNA structure discovered in the 1950s by Watson and Crick
• Researchers learned DNA and the sequence of ‘bases’ was the code
• Location of genes, alleles
• The source of inherited traits
• Human Genome Project has sequenced human DNA – offering an

understanding of the location of particular genes on chromosomes and the traits the genes provide code for

• Genome: organism's complete set of DNA, including all of its genes.
• Genes, segments of DNA, contain the codons: “recipe” or blueprints, for

synthesis of proteins

Vocabulary of Genetics

• Diploid number (46) of chromosomes in all cells except gametes
• 1 pair of sex chromosomes that determine genetic sex (XX =

female, XY = male)

• 22 pairs of autosomes that guide expression of most other

traits

• Autosomes look alike and carry genes for the same traits

but not necessarily the same expression of the trait

• Karyotype: diploid chromosomal complement displayed in

homologous pairs

• Genetic screening and genetic counseling
• Analysis of the pedigree
• Fetal testing: amniocentesis, chorionic villus sampling

Genetically female Notice not all chromosomes are identical in size or shape

Genetically male

Gene Pairs: Alleles

Alleles are genes that occur at same locus (location) on homologous chromosomes

• DNA sequence can be the same or different
• Sequence = groupings of codons that code for amino

acid sequence in a single protein

• “one gene, one protein” – consider the numerous

functions of proteins in A&P

• Homozygous: alleles are same for single trait
• Heterozygous: alleles are different for single trait
• i.e., Allele inherited from mom is NOT identical to

allele inherited from dad but still codes for same trait

Gene Expression

Dominance: one allele masks (suppresses) expression of its recessive partner

• Dominant allele is denoted by capital letter and recessive by same

letter, but in lowercase

• Example: brown eyes is dominant trait, designated as B; blue

eyes is recessive trait designated as b

• Dominant trait is expressed even if other allele codes for

recessive trait

• Example: BB or Bb will result in brown eyes, not blue
• Recessive trait is expressed only if both alleles are recessive
• Example: blue eyes occur only if person has bb
• **There is no inherent benefit or superiority in dominant alleles,

they are just the ones expressed**

• Genotype: genetic makeup of a person for a trait
• For eye color example, person can have three possible genotypes: BB,

Bb, bb

• **Only if the trait is controlled by a single pair of alleles**
• Phenotype: physical expression of genotype
• Person with genotypes BB or Bb will have brown eyes (B is dominant)
• Person with genotype bb will have blue eyes
• Heterozygous person with Bb has genotype for blue eyes, but

phenotypically will have brown eyes

• Few phenotypes can be traced to single gene

Multiple allele inheritance - Most traits determined by multiple alleles

• r by interaction of several gene pairs (polygenetic inheritance)

Gene Expression

Figure 29.4 Genotype and phenotype probabilities resulting from a mating of two heterozygous parents.

Heterozygous female forms two types

• f gametes

Heterozygous male forms two types

• f gametes

Tt female Tt male Possible combinations in offspring 1/4 1/4 1/4 1/4 1/2 1/2 1/2 1/2 T T TT tT Tt tt t t

Using a Punnett Square to predict frequency of genotypes and phenotypes in offspring 1:2:1 genotypic ratio 3:1 phenotypic ratio 25% chance homozygous 50% chance heterozygous

Simple dominance/recessive inheritance not as common as once thought but still…is present in the human genome Dominant traits dictated by dominant alleles

• Simple examples: widow’s peaks, freckles, and dimples
• Dominant disorders are uncommon because most are lethal, and death occurs

before reproductive age

• Exception is Huntington’s disease: not activated until ~ age 40
• Offspring of individual with Huntington’s have a 50% chance of also having disease

Recessive genes may result in the more desirable condition

• Example: normal vision is a recessive trait, whereas astigmatism is a dominant

trait

• Many genetic disorders are inherited as simple recessive traits
• Examples: albinism, cystic fibrosis, and Tay-Sachs disease
• Recessive nature makes them less common in occurrence
• Heterozygotes are carriers of trait, meaning they do not express trait but can

pass it on to offspring

Gene Expression

Table 29.1 Traits Determined by Simple Dominant-Recessive Inheritance

Incomplete dominance

• Heterozygous individuals have intermediate phenotype
• Example: sickling gene causing abnormal hemoglobin
• SS = normal hemoglobin (Hb) made
• Ss = sickle-cell trait: both mutated and normal Hb are made;

person can suffer sickle-cell crisis under prolonged reduction in blood O2

• ss = sickle-cell anemia: makes only mutated Hb; person is more

susceptible to sickle-cell crisis even with short O2 reduction

• FYI - One hypothesis for the presence of the s allele is

that it offers the carrier some protection from the malaria blood parasite!

Gene Expression

Gene Expression

Multiple-allele inheritance

• Genes that exhibit more than two allele forms
• Example: ABO blood groups have three alleles: IA, IB, and I
• Rh status controlled by another gene

Polygene inheritance

• Traits that are result of actions of several gene pairs at different

locations

• The more genes are involved in a trait, the more phenotypic variation

will be seen

• Results in continuous (quantitative) phenotypic variation between two

extremes

• Examples: skin color, eye color, height, intelligence, metabolic rate

Example of polygenic inheritance for skin color

• Alleles for dark skin (ABC) are incompletely dominant over those for

light skin (abc)

• First-generation offspring of AABBCC (very dark)  aabbcc (very light)

cross would result in all heterozygotes with intermediate pigmentation

• Second-generation offspring would have even wider variation in

possible pigmentations, which, if charted, would lead to a bell-shaped curve

Gene Expression

Figure 29.6 Simplified model for polygene inheritance of skin color based on three gene pairs.

Parents First-generation

• ffspring

20/64 15/64 6/64 1/64 1/64 6/64 20/64 15/64 15/64 6/64 1/64 Proportion of second-generation population AaBbCc AaBbCc aabbcc (very light) AABBCC (very dark)

 

P generation: homozygous dominant crossed with homozygous recessive F1 generation: two heterozygous individuals F2 generation: probability

• f particular phenotypes

Inherited traits determined by genes on sex chromosomes

• X chromosomes bear over 1400 genes
• Genes found only on X chromosome are called X-linked genes
• Y chromosomes carry about 200 genes

Note: these genes include those that have nothing to do with the sex

• f the individual
• X-linked recessive alleles are always expressed in males and are

never masked or damped because there is no Y counterpart

• Females must have recessive alleles on both X chromosomes in
• rder to express an X-linked condition
• X-linked recessive conditions are passed from mothers to sons
• Example: hemophilia or red-green color blindness, types of balding
• Can also be passed from mothers to daughters, but females require

two alleles to express

Sex-linked Inheritance

Environmental Factors’ Effect on Gene Expression

In many situations, environment can override or influence gene expression

• Maternal factors can alter normal gene expression during

embryonic development

• Example: teratogenicity of thalidomide
• Embryos developed phenotypes not directed by their genes, but

by the drug

• Environmental factors can also influence gene expression

after birth

• Poor nutrition can affect brain growth, body development, and

height

• Childhood hormonal deficits can lead to abnormal skeletal growth

and proportions

• Lead (Pb)doses in children and impacts to intellectual

development

Supporting Genetic Variability

• 1. Mutations

i. “A mutation is a change that occurs in our DNA sequence, either due to mistakes when the DNA is copied or as the result of environmental factors such as UV light and cigarette smoke. Mutations can occur during DNA replication if errors are made and not corrected in time.” ii. https://courses.lumenlearning.com/wmopen-biology1/chapter/dna- mutations/

• 2. Crossover of homologues during gametogenesis
• 3. Independent assortment of chromosomes during gametogenesis
• 4. Random fertilization of eggs by sperm

Figure 29.3-1 Crossover and genetic recombination.

Hair color genes Eye color genes Homologous chromosomes synapse during prophase of meiosis I. Each chromosome consists of two sister chromatids. h h e e H H E E

Allele for brown hair Allele for blond hair Allele for brown eyes Allele for blue eyes Paternal chromosome Maternal chromosome Homologous pair

Allele for brown hair Allele for blond hair Allele for brown eyes Allele for blue eyes Paternal chromosome Maternal chromosome Homologous pair

One chromatid segment exchanges positions with a homologous chromatid segment—in other words, crossing over occurs, forming a chiasma. Chiasma h h e e E E H H

Figure 29.3-1 Crossover and genetic recombination.

Allele for brown hair Allele for blond hair Allele for brown eyes Allele for blue eyes Paternal chromosome Maternal chromosome Homologous pair

h h e e E E H H The chromatids forming the chiasma break, and the broken-off ends join their corresponding homologues.

Figure 29.3-1 Crossover and genetic recombination.

Figure 29.3-1 Crossover and genetic recombination.

Allele for brown hair Allele for blond hair Allele for brown eyes Allele for blue eyes Paternal chromosome Maternal chromosome Homologous pair

h h e e E E H H At the conclusion of meiosis, each haploid gamete has one

• f the four chromosomes shown. Two of the chromosomes

are recombinant (they carry new combinations of genes). Gamete 1 Gamete 2 Gamete 3 Gamete 4

Disjunction defined

• The normal separation of chromosomes in meiosis I or

sister chromatids in meiosis II is termed disjunction.

• When the separation is not normal, it is called

nondisjunction.

• failure of chromosome pairs to separate properly during meiosis

I or II.

• Why? Simple mistakes occurring infrequently but amplified by

the huge number of gametes produced

• This results in the production of gametes which have either
• too many (result in zygote: trisomy) or
• too few (result in zygote: monosomy) of a particular chromosome

Nondisjunction & Karyotypes

Nondisjunction

The result of this error is gametes with too few or too many chromosomes. An extra or missing chromosome is a common cause of genetic disorders (spontaneous abortions, stilbirths, birth defects, syndromes).

Nondisjunction Causes Aneuploidy

• Aneuploidy is an abnormal number of chromosomes, and

is a type of chromosome abnormality.

• Chromosome abnormalities occur in 1 of 160 live births.
• Most cases of aneuploidy result in termination of the

developing fetus, but there can be cases of live birth

• the most common extra chromosomes among live births are 21,

18 and 13.

• Zygotes:
• Monosomy 2n-1
• Trisomy 2n+1

Human Conditions Resulting from Nondisjunction Syndrome or Condition Anomoly Location Characteristics

Patau trisomy 13 Physical abnormalities, only 5-10% live past first year http://ghr.nlm.nih.gov/condition/trisomy-13 Edward trisomy 18 Physical abnormalities, only 5-10% live past first year http://ghr.nlm.nih.gov/condition/trisomy-18 Down trisomy 21 Intellectual, developmental, and physical symptoms https://www.nichd.nih.gov/health/topics/down/conditioninfo/symptoms Klinefelter trisomy sex XXY male (XXXY, XXXXY possible), Physical and intellectual development issues, small testes and reduced testosterone production, development of male sex characteristics delayed or incomplete including abnormalities of genitals, developmental delays, learning disabilities http://ghr.nlm.nih.gov/condition/klinefelter-syndrome Turner monosomy sex XO female, Physical issues, reduced fertility, low estrogen, heart defects, developmental delays and disabilities http://ghr.nlm.nih.gov/condition/turner-syndrome Triple X trisomy sex XXX female, Increased risk of learning disabilities and development of language and motor skills http://ghr.nlm.nih.gov/condition/triple-x-syndrome 47-XYY trisomy sex No significant effects on male sex characteristic development, but learning disabilities, delayed skill development, behavioral/social/emotional difficulties an extra Y chromosome in males http://ghr.nlm.nih.gov/condition/47xyy-syndrome