Genetics: Analysis and Principles Robert J. Brooker CHAPTER 5 NON-MENDELIAN INHERITANCE

INTRODUCTION Mendelian inheritance patterns involve genes that Directly influence the outcome of an organism’s traits and Obey Mendel’s laws Most genes in eukaryotic species follow a Mendelian pattern of inheritance However, there are many that don’t Linkage can be considered as non-Mendelian inheritance

INTRODUCTION Patterns of inheritance that deviate from a Mendelian pattern: Maternal effect and epigenetic inheritance Involve genes in the nucleus Extranuclear inheritance Involves genes in organelles other than the nucleus Mitochondria Chloroplasts

5.1 MATERNAL EFFECT Maternal effect refers to an inheritance pattern for certain nuclear genes in which the genotype of the mother directly determines the phenotype of her offspring This phenomenon is due to the accumulation of gene products that the mother provides to her developing eggs

The first example of a maternal effect gene was discovered in the 1920s by Boycott He was studying morphological features of the water snail, Limnaea peregra In this species, the shell and internal organs can be arranged in one of two directions Right-handed (dextral) Left-handed (sinistral) The dextral orientation is more common and dominant The snail’s body plan curvature depends on the cleavage pattern of the egg immediately after fertilization Figure 5.1 describes Boycott’s experiment

Reciprocal cross Figure 5.1  reciprocal cross is a breeding experiment designed to test the role of parental sex on a given inheritance pattern. All parent organisms must be true breeding to properly carry out such an experiment. In one cross, a male expressing the trait of interest will be crossed with a female not expressing the trait. In the other, a female expressing the trait of interest will be crossed with a male not expressing the trait. Reciprocal cross A 3:1 phenotypic ratio would be predicted by a Mendelian pattern of inheritance Figure 5.1

Alfred Sturtevant later explained the inconsistency with Mendelian inheritance Snail coiling is due to a maternal effect gene that exists as dextral (D) and sinistral (d) alelles The phenotype of the offspring depended solely on the genotype of the mother His conclusions were drawn from the inheritance patterns of the F2 and F3 generations

Fig. 7.1(TE Art) Parental generation DD dd dd DD F1 generation Dd Dd Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 7.1(TE Art) Parental generation DD dd dd DD F1 generation Dd Dd All dextral All sinistral F2 generation Males and females 1 DD 2 Dd 1 dd All dextral Cross to each other F3 generation Males and females 1 sinistral 3 dextral

DD or Dd mothers produce dextral offspring Note that the phenotype of each generation depends on the maternal genotype of the previous generation DD or Dd mothers produce dextral offspring dd mothers produce sinistral offspring The phenotype of the progeny is determined by the mother’s genotype NOT phenotype The genotypes of the father and offspring do not affect the phenotype of the offspring

The gene products are a reflection of the genotype of the mother They are transported to the cytoplasm of the oocyte where they persist for a significant time after the egg has been fertilized Thus influencing the early developmental stages of the embryo Figure 5.2

D gene products cause egg cleavage that promotes a right-handed body plan Figure 5.2

Even if the egg is fertilized by sperm carrying the D allele d gene products cause egg cleavage that promotes a left-handed body plan Even if the egg is fertilized by sperm carrying the D allele The sperm’s genotype is irrelevant because the expression of the sperm’s gene would be too late Figure 5.2

Maternal effect genes encode RNA or proteins that play important roles in the early steps of embryogenesis For example Cell division Cleavage pattern In Drosophila, geneticists have identified several dozen maternal effect genes These have profound effects on the early stages of development

5.2 EPIGENETIC INHERITANCE Epigenetic inheritance refers to a pattern in which a modification occurs to a nuclear gene or chromosome that alters gene expression However, the expression is not permanently changed over the course of many generations Epigenetic changes are caused by DNA and chromosomal modifications These can occur during oogenesis, spermatogenesis or early embryonic development

Dosage Compensation The purpose of dosage compensation is to offset differences in the number of active sex chromosomes Dosage compensation has been studied extensively in mammals, Drosophila and Caenorhabditis elegans Depending on the species, dosage compensation occurs via different mechanisms Refer to Table 5.1

Fig. 5.3 Barr body is a highly condensed X chromosome

The mechanism of X inactivation, also known as the Lyon hypothesis, is schematically illustrated in Figure 5.4 The example involves a white and black variegated coat color found in certain strains of mice A female mouse has inherited two X chromosomes One from its mother that carries an allele conferring white coat color (Xb) One from its father that carries an allele conferring black coat color (XB)

While those from this will produce a patch of black fur The epithelial cells derived from this embryonic cell will produce a patch of white fur At an early stage of embryonic development While those from this will produce a patch of black fur Figure 5.4

During X chromosome inactivation, the DNA becomes highly compacted Most genes on the inactivated X cannot be expressed When this inactivated X is replicated during cell division Both copies remain highly compacted and inactive In a similar fashion, X inactivation is passed along to all future somatic cells Another example of variegated coat color Is found in calico cats Refer to Figure 5.3b

5.3 EXTRANUCLEAR INHERITANCE Extranuclear inheritance refers to inheritance patterns involving genetic material outside the nucleus The two most important examples: mitochondria and chloroplasts These organelles are found in the cytoplasm Extranuclear inheritance = cytoplasmic inheritance

The genetic material of mitochondria and chloroplasts is located in a region called the nucleoid Refer to Figure 5.14 The genome is composed of a single circular chromosome containing double-stranded DNA

In general, mitochondrial genomes are Fairly small in animals Intermediate in size in fungi, algae and protists Fairly large in plants

Nuclear encoded genes in red

The main function of mitochondria is oxidative phosphorylation A process used to generate ATP (adenosine triphosphate) ATP is used as an energy source to drive cellular reactions The genetic material in mitochondria is referred to as mtDNA The human mtDNA consists of only 17,000 bp (Figure 5.14) It carries relatively few genes rRNA and tRNA genes 13 genes that function in oxidative phosphorylation Note: Most mitochondrial proteins are encoded by genes in the nucleus These proteins are made in the cytoplasm, then transported into the mitochondria

Necessary for synthesis of proteins inside the mitochondrion Function in oxidative phosphorylation Figure 5.14

The main function of chloroplasts is photosynthesis The genetic material in chloroplasts is referred to as cpDNA It is typically about 10 times larger than the mitochondrial genome of animal cells The cpDNA of tobacco plant consists of 156,000 bp It carries between 110 and 120 different genes rRNA and tRNA genes Many genes that are required for photosynthesis As with mitochondria, many chloroplast proteins are encoded by genes in the nucleus These proteins contain chloroplast-targeting signals that direct them from the cytoplasm into the chloroplast

Figure 5.15 A genetic map of the tobacco chloroplast genome Genes designated ORF (open reading frame) encode polypeptides with unknown functions Figure 5.15 A genetic map of the tobacco chloroplast genome

Extranuclear Inheritance Produces Non-Mendelian Results in Reciprocal Crosses In 1909, Carl Correns discovered a trait that showed a non-Mendelian pattern of inheritance involving pigmentation in Mirabilis jalapa (the four-o’clock plant). Leaves can be green, white, or variegated with both green and white sectors. Correns demonstrated that the pigmentation of the offspring depended solely on the maternal parent Refer to Figure 5.17

If the female parent had white pigmentation, all offspring had white leaves. If the female was green, all offspring were green. When the female was variegated, the offspring could be green, white, or variegated. The pattern of inheritance observed by Correns is a type of extranuclear inheritance called maternal inheritance.

Chloroplasts are a type of plastid that makes chlorophyll, a green photosynthetic pigment. Maternal inheritance occurs because the chloroplasts are inherited only through the cytoplasm of the egg. The pollen grains of M. jalapa do not transmit chloroplasts to the offspring.

The phenotypes of leaves can be explained by the types of chloroplasts within the leaf cells. The green phenotype, which is the wild-type condition, is due to the presence of normal chloroplasts that make green pigment. The white phenotype is due to a mutation in a gene within the chloroplast DNA that diminishes the synthesis of green pigment. A cell may contain both types of chloroplasts, a condition known as heteroplasmy.

How does a variegated phenotype occur?