Heredity Part 2

Genetics

• The study of the mechanism of heredity• Nuclei of all human cells (except gametes) contain 46

chromosomes • Sex chromosomes determine the genetic sex

(XX = female, XY = male)• Karyotype – the diploid chromosomal complement

displayed in homologous pairs• Genome – genetic (DNA) makeup represents two sets of

genetic instructions – one maternal and the other paternal

Alleles• Matched genes at the same locus on

homologous chromosomes• Homozygous – two alleles controlling a single

trait are the same• Heterozygous – the two alleles for a trait are

different• Dominant – an allele masks or suppresses the

expression of its partner• Recessive – the allele that is masked or

suppressed

Genotype and Phenotype

• Genotype – the genetic makeup• Phenotype – the way one’s genotype is

expressed

Segregation and Independent Assortment

• Chromosomes are randomly distributed to daughter cells

• Members of the allele pair for each trait are segregated during meiosis

• Alleles on different pairs of homologous chromosomes are distributed independently

Segregation and Independent Assortment

• The number of different types of gametes can be calculated by this formula:

2n, where n is the number of homologous pairs• In a man’s testes, the number of gamete types

that can be produced based on independent assortment is 223, which equals 8.5 million possibilities

Independent Assortment

Figure 29.2

Crossover

• Homologous chromosomes synapse in meiosis I• One chromosome segment exchanges positions

with its homologous counterpart • Genetic information is exchanged between

homologous chromosomes• Two recombinant chromosomes are formed

Crossover

Figure 29.3.1

Crossover and Genetic Recombination

Figure 29.3.2

Random Fertilization

• A single egg is fertilized by a single sperm in a random manner

• Considering independent assortment and random fertilization, an offspring represents one out of 72 trillion (8.5 million 8.5 million) zygote possibilities

Dominant-Recessive Inheritance

• Reflects the interaction of dominant and recessive alleles

• Punnett square – diagram used to predict the probability of having a certain type of offspring with a particular genotype and phenotype

• Example: probability of different offspring from mating two heterozygous parentsT = tongue roller and t = cannot roll tongue

Dominant-Recessive Inheritance

Figure 29.4

Dominant-Recessive Inheritance

• Examples of dominant disorders: achondroplasia (type of dwarfism) and Huntington’s disease

• Examples of recessive conditions: albinism, cystic fibrosis, and Tay-Sachs disease

• Carriers – heterozygotes who do not express a trait but can pass it on to their offspring

Incomplete Dominance• Heterozygous individuals have a phenotype

intermediate between homozygous dominant and homozygous recessive

• Sickling gene is a human example when aberrant hemoglobin (Hb) is made from the recessive allele (s)SS = normal Hb is madeSs = sickle-cell trait (both aberrant andnormal Hb is made)

ss = sickle-cell anemia (only aberrant Hb is made)

Incomplete dominance in snapdragon color

Incomplete dominance in human hypercholesterolemia

Multiple-Allele Inheritance

• Genes that exhibit more than two alternate alleles

• ABO blood grouping is an example• Three alleles (IA, IB, i) determine the ABO blood

type in humans• IA and IB are codominant (both are expressed if

present), and i is recessive

ABO Blood Groups(codominant)

Table 29.2

Multiple alleles for the ABO blood groups

Sex-Linked Inheritance

• Inherited traits determined by genes on the sex chromosomes

• X chromosomes bear over 2500 genes; Y chromosomes carry about 15 genes

• X-linked genes are:– Found only on the X chromosome – Typically passed from mothers to sons– Never masked or damped in males since there is

no Y counterpart

Polygene Inheritance

• Depends on several different gene pairs at different loci acting in tandem

• Results in continuous phenotypic variation between two extremes

• Examples: skin color, eye color, and height, metabolic rate,inteligence.

Polygenic Inheritance of Skin Color• Alleles for dark skin (ABC) are incompletely

dominant over those for light skin (abc)• The first generation offspring each have three

“units” of darkness (intermediate pigmentation)

• The second generation offspring have a wide variation in possible pigmentations

Polygenic Inheritance of Skin Color

Figure 29.5

A model for polygenic inheritance of skin color

Sickle cell disease, multiple effects of a single human gene

Environmental Influence on Gene Expression

• Phenocopies – environmentally produced phenotypes that mimic mutations

• Environmental factors can influence genetic expression after birth– Poor nutrition can effect brain growth, body

development, and height– Childhood hormonal deficits can lead to abnormal

skeletal growth

Thank you

Genomic Imprinting• The same allele can have different effects

depending upon the source parent• Deletions in chromosome 15 result in:– Prader-Willi syndrome if inherited from the father– Angelman syndrome if inherited from the mother

• During gametogenesis, certain genes are methylated and tagged as either maternal or paternal

• Developing embryos “read” these tags and express one version or the other

Extrachromosomal (Mitochondrial) Inheritance

• Some genes are in the mitochondria• All mitochondrial genes are transmitted by the

mother• Unusual muscle disorders and neurological

problems have been linked to these genes

Genetic Screening, Counseling, and Therapy

• Newborn infants are screened for a number of genetic disorders: congenital hip dysplasia, imperforate anus, and PKU

• Genetic screening alerts new parents that treatment may be necessary for the well-being of their infant

• Example: a woman pregnant for the first time at age 35 may want to know if her baby has trisomy-21 (Down syndrome)

Carrier Recognition• Identification of the heterozygote state for a

given trait• Two major avenues are used to identify

carriers: pedigrees and blood tests• Pedigrees trace a particular genetic trait

through several generations; helps to predict the future

• Blood tests and DNA probes can detect the presence of unexpressed recessive genes

• Sickling, Tay-Sachs, and cystic fibrosis genes can be identified by such tests

Pedigree Analysis

Figure 29.6

Fetal Testing• Is used when there is a known risk of a genetic

disorder• Amniocentesis – amniotic fluid is withdrawn

after the 14th week and sloughed fetal cells are examined for genetic abnormalities

• Chorionic villi sampling (CVS) – chorionic villi are sampled and karyotyped for genetic abnormalities

Fetal Testing

Figure 29.7

Human Gene Therapy• Genetic engineering has the potential to

replace a defective gene• Defective cells can be infected with a

genetically engineered virus containing a functional gene

• The patient’s cells can be directly injected with “corrected” DNA

• What makes a dominant gene dominant?• Usually, the masking effect is done by virtue of the fact

that the recessive gene has a loss of some function that the dominant gene has. For example, in the case of ABO blood types, the O type is recessive because it does not produce any antigens or antibodies, whereas A and B types (which are co-dominant) do. Or, in the case of eye color, there is a complete loss of pigment in blue-eyed people, therefore to express the phenotype, both copies of the gene (after all, humans are diploid) must have that same loss of function.

• What makes an allele recessive (the opposite of dominant) is if it DOESN'T code for something, or if it codes for an ALTERED something. In the case of blue eyes, there is no gene that says "make melanin!", so your eyes don't get dark, they stay light blue or gray. In the case of green eyes you have a gene that says "make some really weird melanin!" and so your eyes get a green pigment instead of a normal dark pigment.